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THoughts on an 18" rim on my Formula

Old 07-08-2019, 10:21 PM
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THoughts on an 18" rim on my Formula

So I was told that a 5x120mm 18x8 rim will fit on my car. I am looking at these:
https://www.summitracing.com/parts/UWC-4328812SBX40

Some questions:
1. Does 5x120mm fit the third gens?
2. I am thinking a 245/50 18" tire, sound right, any concerns with rubbing on turns, etc?
3. Any needs to adjust the speedomete, if so how?

Thanks gents!
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Old 07-08-2019, 10:53 PM
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Re: THoughts on an 18" rim on my Formula

People (lots of people actually) have used the 5 x 120 mm bolt pattern but it's not wise in my opinion. I once got a wheel manufacturer to share dimensions and tolerancing of the holes in the wheel and it was waaaaaaaaaaay tighter specs than people commonly think. There was no overlap of tolerance between the 5 x 120 and the 5 x 4-3/4 with that particular manufacturer.

18" wheels and tires are much heavier than 17", and the cost of 18" tires is double too. Car will accelerate noticeably slower and keep picking your wallet every time you buy tires. Just something to consider.
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Old 07-08-2019, 10:57 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by QwkTrip View Post
People (lots of people actually) have used the 5 x 120 mm bolt pattern but it's not wise in my opinion. I once got a wheel manufacturer to share dimensions and tolerancing of the holes in the wheel and it was waaaaaaaaaaay tighter specs than people commonly think. There was no overlap of tolerance between the 5 x 120 and the 5 x 4-3/4 with that particular manufacturer.

18" wheels and tires are much heavier than 17", and the cost of 18" tires is double too. Car will accelerate noticeably slower and keep picking your wallet every time you buy tires. Just something to consider.
Well thats an honest answer for sure. Any yes the tires can get expensive. Maybe a 17" alternative. They to make a 1" adapter for 5x4 3/4 to 5x120mm. Maybe thats the way to go.

Last edited by Hawkeye1980; 07-08-2019 at 11:09 PM.
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Old 07-08-2019, 11:23 PM
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Re: THoughts on an 18" rim on my Formula

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Old 07-09-2019, 11:40 AM
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Re: THoughts on an 18" rim on my Formula

//<86TA>\\ re-drilled the PCD on some BMW wheels, 120mm, so he could use them on his 92 Firebird
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Old 07-09-2019, 05:39 PM
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Re: THoughts on an 18" rim on my Formula

The wheel you linked to has a positive 40mm offset, so you need a spacer about 1.5" thick to fit it. Use an adapter which converts the 5x120 to 5x4.75 and you are all set.

245/40/18 is the height you want and is more common than the 245/50/18.
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Old 07-09-2019, 07:56 PM
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Re: THoughts on an 18" rim on my Formula

Im running Apex Arc-8 18x10 25mm offset on all 4 corners of my 85 IROC. I use BMW 5 x 120. The center bore is 72.56mm which leaves about 1mm clearance for the wheel to locate. This is not an issue as our cars are not designed as hub-centric. Im using 12mm spacers - THRU spacers, none of this separate extra bolt/stud stuff.
The 4.75 BC our cars have is 120.65mm, typically .325mm on the radius, or about .013 difference from the BMW bolt circle. This is smaller than a typical manufacturing tolerance, In fact Id be impressed if any wheel or Hub/rotor manufacturer could hit or in fact limit themselves to .010 with regularity - and as such .003 would be an acceptable deviation for a saleable part. But it wouldnt even get as far as a non-conformance on the manufacturing side as geometric dimensioning and tolerancing (GD&T) typically allows a bonus tolerance to a pattern like this. So .65mm on a 120mm hole pattern is a non-issue.
Short version: ya, its good. Dont worry about it. Get the back spacing figured out for an 8 rim...But:
Addenum: the center cap locating surface at the outer edge of the middle/center of the rim might interfere with the snout of the front hub. And the dust cap on the rotor/hub will NOT allow a wheel center cap to be installed with these specific wheels. (For my application, with the 12mm spacers there is no interference here, but I can not use center caps on the wheels as the dust cap protrudes through there.
Also - wheel weight will vary by manufacturers, mine are within a few ounces of the leading lightweight 17 wheel. AND tire choice is better in 18, and you can run more back spacing because the steering knuckle fits inside the rim with 18 wheels - it hits the rim on a 17 wheel, limiting your backspacing and therefore wheel choice.


happy wheel fitting!

Last edited by RoadRace; 07-09-2019 at 08:04 PM.
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Old 07-09-2019, 08:23 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
This is not an issue as our cars are not designed as hub-centric.
Yes, they ARE hubcentric. Almost all wheels from production cars are hubcentric
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Old 07-09-2019, 08:23 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
Im running Apex Arc-8 18x10 25mm offset on all 4 corners of my 85 IROC. I use BMW 5 x 120. The center bore is 72.56mm which leaves about 1mm clearance for the wheel to locate. This is not an issue as our cars are not designed as hub-centric. Im using 12mm spacers - THRU spacers, none of this separate extra bolt/stud stuff.
The 4.75 BC our cars have is 120.65mm, typically .325mm on the radius, or about .013 difference from the BMW bolt circle. This is smaller than a typical manufacturing tolerance, In fact Id be impressed if any wheel or Hub/rotor manufacturer could hit or in fact limit themselves to .010 with regularity - and as such .003 would be an acceptable deviation for a saleable part. But it wouldnt even get as far as a non-conformance on the manufacturing side as geometric dimensioning and tolerancing (GD&T) typically allows a bonus tolerance to a pattern like this. So .65mm on a 120mm hole pattern is a non-issue.
Short version: ya, its good. Dont worry about it. Get the back spacing figured out for an 8 rim...But:
Addenum: the center cap locating surface at the outer edge of the middle/center of the rim might interfere with the snout of the front hub. And the dust cap on the rotor/hub will NOT allow a wheel center cap to be installed with these specific wheels. (For my application, with the 12mm spacers there is no interference here, but I can not use center caps on the wheels as the dust cap protrudes through there.
Also - wheel weight will vary by manufacturers, mine are within a few ounces of the leading lightweight 17 wheel. AND tire choice is better in 18, and you can run more back spacing because the steering knuckle fits inside the rim with 18 wheels - it hits the rim on a 17 wheel, limiting your backspacing and therefore wheel choice.


happy wheel fitting!
I ran the same wheels, great setup.

I will disagree with the 120 vs 120.65 thing as usual. The manufactures tolerance is much lower, and its beyond crazy to say otherwise. While it's close enough that the nuts and lugs will seat enough to keep the wheels on, it's not something I would ever recommend to anyone.

And the BMW bore is 72.6, stock is 70.3. There are hub centric rings available to make the fit perfect. I used 12mm hubcentric spacers with the 70.3 to 72.6 rings on the inside for the rear. Custom hubs on the front so they fit without any spacers.
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Old 07-09-2019, 08:29 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by scooter View Post
Yes, they ARE hubcentric. Almost all wheels from production cars are hubcentric
In the 80s-90s I worked in the aftermarket wheel industry. it was widely accepted that GM was not hub centric at that time.
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Old 07-09-2019, 08:35 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
In the 80s-90s I worked in the aftermarket wheel industry. it was widely accepted that GM was not hub centric at that time.
Then how come my GTA wheels fit so tight on the register on the hubs and axles that any bit of corrosion can cause them to get stuck on? My 89 Grand Am was hub, my 69 C10 is hub, my 73 and 71 Camaro are hub, all the 90's Chevy trucks I've worked on are hub......Nay, just bad info. The hub is what carries the weight of the vehicle to the wheel. With the exception of big trucks, almost all vehicles are hub centric
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Old 07-09-2019, 08:38 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by //<86TA>\\ View Post
I ran the same wheels, great setup.

I will disagree with the 120 vs 120.65 thing as usual. The manufactures tolerance is much lower, and its beyond crazy to say otherwise. While it's close enough that the nuts and lugs will seat enough to keep the wheels on, it's not something I would ever recommend to anyone.

And the BMW bore is 72.6, stock is 70.3. There are hub centric rings available to make the fit perfect. I used 12mm hubcentric spacers with the 70.3 to 72.6 rings on the inside for the rear. Custom hubs on the front so they fit without any spacers.
All good! I completely support your difference in opinion - awesome!
Ive worked in aerospace and defence engineering, design and QA for 20 years. While I will applaud any manufacturer that tries to sell a wheel saying they use 4 decimal place tolerance I know for a fact otherwise. I work closely with Calibration and CMM and I can say with certainty that my original statement is solid.
Fact is that wheel manufacturing can not control the mating part and have to have allowances to make their parts work with everybodys, and while the OEMs can hit tight numbers, the off-shore aftermarket hubs that will sometimes be replacing the OEM parts are not governed by the same set of regulations - enter the larger tolerance required to maintain the life of the wheels they are selling.
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Old 07-09-2019, 08:44 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by scooter View Post
Then how come my GTA wheels fit so tight on the register on the hubs and axles that any bit of corrosion can cause them to get stuck on? My 89 Grand Am was hub, my 69 C10 is hub, my 73 and 71 Camaro are hub, all the 90's Chevy trucks I've worked on are hub......Nay, just bad info. The hub is what carries the weight of the vehicle to the wheel. With the exception of big trucks, almost all vehicles are hub centric
The dissimilar metal electrolitic effect with Alum wheels that propagates corrosion usually starts at the hub where the there is a small clearance - but there is a clearance. Its that could be a possible reason for your wheels to be so tight...mine never were, but yours could also be on the tight side of the manufacturer tolerance. The steel wheels had an irregular bore ID so they could try and locate on the hub as well. Using the hub to help distribute load does not indicate its also used as a datum.

To say that the 1/4 hub flange carries the entire load of an entire vehicle might be considered incorrect

Last edited by RoadRace; 07-09-2019 at 08:49 PM.
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Old 07-09-2019, 08:52 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
The dissimilar metal electrolitic effect with Alum wheels that propagates corrosion usually starts at the hub where the there is a small clearance - but there is a clearance. The steel wheels had an irregular bore ID so they could try and locate on the hub as well. Using the hub to help distribute load does not indicate it’s also used as a datum.

To say that the 1/4 “ hub flange carries the entire load of an entire vehicle might be considered incorrect
Yes, I understand how corrosion develops.

"Automotive applications usually use the hub centric design because the support and centering of the wheel better resists impact loads from being used on typical roads."

The register prevents the stud holes from taking abuse from road , so between that and friction, it is holding the weight of the vehicle, it's actually carrying much more than the weight of the vehicle, so it's not incorrect at all
from here but you must know all this already working for an aftermarket wheel manufacturer
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Old 07-09-2019, 09:05 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by scooter View Post
Yes, I understand how corrosion develops.

"Automotive applications usually use the hub centric design because the support and centering of the wheel better resists impact loads from being used on typical roads."

The register prevents the stud holes from taking abuse from road , so between that and friction, it is holding the weight of the vehicle, it's actually carrying much more than the weight of the vehicle, so it's not incorrect at all
The hub diameter is a single part of the load bearing surfaces that all work together to keep the wheel on. And as I said earlier, the tolerance given on any of the 3 active load bearing surfaces that hold the wheel on the car will affect each other differently, and they MUST work together regardless of who made the parts - enter my original statement regarding tolerances. The hub stub itself - in our cars - takes the least load, with the hub face being number one, and the studs being held in double shear as number 2.

Last edited by RoadRace; 07-09-2019 at 09:08 PM. Reason: Typo and Im sounding like a dick head now...
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Old 07-09-2019, 10:27 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by //<86TA>\\ View Post
The wheel you linked to has a positive 40mm offset, so you need a spacer about 1.5" thick to fit it. Use an adapter which converts the 5x120 to 5x4.75 and you are all set.

245/40/18 is the height you want and is more common than the 245/50/18.
Thanks so much!!! I can do a bunch of stuff but wheels, offset, blah blah escapes me!
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Old 07-09-2019, 10:30 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
Im running Apex Arc-8 18x10 25mm offset on all 4 corners of my 85 IROC. I use BMW 5 x 120. The center bore is 72.56mm which leaves about 1mm clearance for the wheel to locate. This is not an issue as our cars are not designed as hub-centric. Im using 12mm spacers - THRU spacers, none of this separate extra bolt/stud stuff.
The 4.75 BC our cars have is 120.65mm, typically .325mm on the radius, or about .013 difference from the BMW bolt circle. This is smaller than a typical manufacturing tolerance, In fact Id be impressed if any wheel or Hub/rotor manufacturer could hit or in fact limit themselves to .010 with regularity - and as such .003 would be an acceptable deviation for a saleable part. But it wouldnt even get as far as a non-conformance on the manufacturing side as geometric dimensioning and tolerancing (GD&T) typically allows a bonus tolerance to a pattern like this. So .65mm on a 120mm hole pattern is a non-issue.
Short version: ya, its good. Dont worry about it. Get the back spacing figured out for an 8 rim...But:
Addenum: the center cap locating surface at the outer edge of the middle/center of the rim might interfere with the snout of the front hub. And the dust cap on the rotor/hub will NOT allow a wheel center cap to be installed with these specific wheels. (For my application, with the 12mm spacers there is no interference here, but I can not use center caps on the wheels as the dust cap protrudes through there.
Also - wheel weight will vary by manufacturers, mine are within a few ounces of the leading lightweight 17 wheel. AND tire choice is better in 18, and you can run more back spacing because the steering knuckle fits inside the rim with 18 wheels - it hits the rim on a 17 wheel, limiting your backspacing and therefore wheel choice.


happy wheel fitting!

All GREAT advise, thanks man!
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Old 07-09-2019, 10:36 PM
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Re: THoughts on an 18" rim on my Formula

So at this point should I go 17" or 18"? I can get this style in both sizes. I picked an 8" width being stock is 16x8, should I go wider? Tires cost more I was told in 18", and there is a power loss. Plus does the speedo need any kind of adjustment. Memory serves that if I change rim size, must adapt the speedo. This is the style I want, 5 spoke, black. Just want to nail down which size, witdth and proper spacing, etc.

Thanks again for all the insight, very helpful!
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Old 07-09-2019, 10:47 PM
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Re: THoughts on an 18" rim on my Formula

Whether 17 or 18 there is no right or wrong answer here. Just choose your heart's desire.

From a practical standpoint I would choose a size that allows you to use a very common tire size that will be in production a long time. There are gobs of strange 18 tire sizes, some of which are already going out of production. I have a friend with a Porsche Cayman that has this problem right now. I think the same thing is happening to the C5 Corvette guys.
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Old 07-09-2019, 10:52 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by QwkTrip View Post
Whether 17 or 18 there is no right or wrong answer here. Just choose your heart's desire.

From a practical standpoint I would choose a size that allows you to use a very common tire size that will be in production a long time. There are gobs of strange 18 tire sizes, some of which are already going out of production. I have a friend with a Porsche Cayman that has this problem right now. I think the same thing is happening to the C5 Corvette guys.
good to know. Decisions... Decisions.....
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Old 07-09-2019, 11:15 PM
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Re: THoughts on an 18" rim on my Formula

Yep, dreamin' and scheme'n is fun stuff.
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Old 07-09-2019, 11:28 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by QwkTrip View Post
Yep, dreamin' and scheme'n is fun stuff.

YES it is!

So here is where I am at:
Wheels:
https://www.tirerack.com/wheels/Whee...kageFlow=false

Tires:
https://www.tirerack.com/tires/tires...cleSearch=true

1.5" spacers 5x4.75 to 5x122mm.

So now I wonder about the speedometer accuracy, and how it may affect acceleration of the car.
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Old 07-09-2019, 11:54 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by Hawkeye1980 View Post
and how it may affect acceleration of the car.
Well you can't screw it up as much as my friend did. He ran 11's with an LS2 swap. Then he went and installed a Ford 9-inch rear, huge 14" brakes with billet 6 piston calipers, and 18 x 14" wheels (that's not a typo). I'm like, what the hell are you doing???? Sure enough, the car slowed down so much it's just average again!
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Old 07-10-2019, 08:16 AM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
The “hub stub” itself - in our cars - takes the least load, with the hub face being number one, and the studs being held in double shear as number 2.


When did wheel studs become double shear?

Last edited by scooter; 07-10-2019 at 08:20 AM.
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Old 07-10-2019, 10:50 AM
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Re: THoughts on an 18" rim on my Formula

I always ask my self, when people are looking to buy wheels why would they buy wheels they know will require spacers or adapters? If you are spending the money why not buy wheels that fit.

But If you really like those wheels
1. Does 5x120mm fit the third gens?
As discussed above, no but you need spacers anyways so you can get ones that adapt the bolt pattern.

2. I am thinking a 245/50 18" tire, sound right, any concerns with rubbing on turns, etc?
Your car had 245/50 r16 factory tires so your best tire size not to scrub will be 245/40R18 on a 8 inch wide wheel or if you do a 9 inch wide wheel I would do 275/35R18 (again as Qwktrip said make sure you can find the tires you want in that size)
https://tiresize.com/tires/Pontiac/F.../1989/Formula/ will tell you the alternative sizes and how they compare to stock.

3. Any needs to adjust the speedomete, if so how?
Dakota digital makes a conversion box since you have an electronic speedometer in your car, but the best is to just make a mental note of the differences using the GPS on your phone. You can also make a conversion box using an Arduino if you are handy with electronics. I will actually have a write up and publish the code for this in a few months as I'm playing around with making an old firebird cluster work with racing games LOL *yea I needed a new hobby* -- unfortunately all the "internet knowledge" shows you to use a low side circuit to translate the 5vdc to 12vdc signal and you actually need a high side switching circuit so I had to design my own circuit. Arduino and parts will run you about $50, but I think when I'm done it will allow you to calibrate both the speedometer and tachometer, it would also allow you to print your own gauge face and calibrate the needle sweep to match. -- no promises on turn around on this though I have alot going on.
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Old 07-10-2019, 01:36 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by RoadRace View Post
All good! I completely support your difference in opinion - awesome!
I’ve worked in aerospace and defence engineering, design and QA for 20 years. While I will applaud any manufacturer that tries to sell a wheel saying they use 4 decimal place tolerance I know for a fact otherwise. I work closely with Calibration and CMM and I can say with certainty that my original statement is solid.
Fact is that wheel manufacturing can not control the mating part and have to have allowances to make their parts work with everybody’s, and while the OEMs can hit tight numbers, the off-shore aftermarket hubs that will sometimes be replacing the OEM parts are not governed by the same set of regulations - enter the larger tolerance required to maintain the life of the wheels they are selling.
I would agree that in many cases of manufacturing that tolerances are not as tight as 4 decimal places out but when dealing with a tapered bore isnt it more critical? Take this wheel hub example. If you center up a 5x120 wheel on a 5x120.65 hub, the taper of the lug nut will hit the side of the tapered bore long before the nut is tightened anything more than finger tight. The gap between the nut and the bore is rather large, about the width of standard #2 pencil lead. My old thread covering the PCD correction has the picture from my install. So while the difference in pattern sounds small on paper, it is in fact quite significant. I can keep a closer tolerance with a tape measure and hand drill. I have never seen a lug nut from any other wheel and hub assembly on any vehicle I have ever worked on engage as poorly. They all for that matter mate up perfectly by hand before needing to tighten them further with a wrench. Now I dont work on cars for a living but I've done enough mechanical work to feel very confident in my assessment.

I'm not trying to be a smart *** or anything, but this topic is one of a few which really peves me and I feel there is a lot of misinformation being spread and not enough people take the time to do their own research.

For the record, we are talking about .0127" difference from center of the hub to center of each lug bore. That's a huge tolerance.

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Old 07-10-2019, 10:42 PM
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Re: THoughts on an 18" rim on my Formula

I always ask my self, when people are looking to buy wheels why would they buy wheels they know will require spacers or adapters? If you are spending the money why not buy wheels that fit.

Wheels will be the one thing that has initial impact and people see that will identify a car as yours. If personalization is important, spend some time and a little bit more money and get something that really buzzes your nuts. Chances are, the look will also be appealing to all those people who wanted to, but ended up with cookie cutters.... Formys can really be stanced out, you need wheels to do it.
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Old 07-10-2019, 11:51 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by Aviator857 View Post
If you are spending the money why not buy wheels that fit.
Because it's not very affordable. Our needed bolt pattern and wheel specs have all but been abandoned except for a few choices that are ugly shizzle or OE replicas. You have to buy custom wheels that are $3K -$6K.
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Old 07-11-2019, 08:47 PM
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Re: THoughts on an 18" rim on my Formula

Thanks Aviator, again great info! Much appreciated!
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Old 07-11-2019, 08:49 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by QwkTrip View Post
Because it's not very affordable. Our needed bolt pattern and wheel specs have all but been abandoned except for a few choices that are ugly shizzle or OE replicas. You have to buy custom wheels that are $3K -$6K.


yes I agree. not much to choose from in our stud pattern. I just like that 5 spoke look and think it would look bad @$$ on the car.
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Old 07-11-2019, 08:55 PM
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Re: THoughts on an 18" rim on my Formula

You don't need to ask people what tire size will keep the speedometer from changing. Just go to the tire manufacturer's website and they'll list all the tire sizes in a chart and you can see the diameter of all the sizes. Aim for something close to 26 inch diameter.
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Old 07-11-2019, 09:11 PM
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Re: THoughts on an 18" rim on my Formula

Originally Posted by QwkTrip View Post
You don't need to ask people what tire size will keep the speedometer from changing. Just go to the tire manufacturer's website and they'll list all the tire sizes in a chart and you can see the diameter of all the sizes. Aim for something close to 26 inch diameter.
got it! thanks!
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