I am not sure if this is the right board for this question, but nobody I have talked to seems to have the answer to this question, so here goes... I just recently purchased an 85 trans-am and it has three diffrent types of wheel on it! none of them are stock(one of them is an old jaguar wheel if you can believe that!) I have decided to buy some aftermarket wheels for it and would like to know what the max tire size and offset I can use without them rubbing during cornering or with the steering wheel at full lock. I would like to go to a 19 inch wheel with 30 to 40 series tires. (narrower in the front) I will be lowering it 2 1/2 inches so tire/wheel fit is really important! I hope someone has the answer to this question.... Thanks!

 

WELCOME to the board..............

It really depends on what rim you want rather than going just by size. I wouldn't suggest going wider than the following.

9.5" rim with a 5.5" bs (backspace) on the front and 10" rim with a 6" bs on the rear. With this you could put a 275 front tire and a 295 rear tire. Sometimes the front rubs and somethmes it doesn't. Depends on the car.

Most common sizes is a 8" on the front with a 4.5" bs and 9.5" on the rear with a 5.5" bs running a 245 front and 275 rear tire.

Good luck.

 

Thank you the project! I appreciate it! Noone seems to know the answers to these questions! I will probably try to go with a 295/30/19 in the rear then and a 255/30/19 in the front.(I dont want any rubbing) My 90 camaro is my nightmare project, the trans am is (hopefully) going to be my daily driver. Anyway, thanks a lot!!!

 

ill get back to you tomorrow

heres what i run
295 30 19 rear
255 40 18 front so i think i can help you with sizing and offset since i calc'd it myself for my car and it worked out nicely.

 

Hey...GodSpeedGTA....can you post a pic of your car with these wheels?| I would love to see what this looks like. If it looks good I may do the same!!

 

Go here 3/4 of the page down youll find plenty of pics

https://www.thirdgen.org/forums/fire...birds-t-8.html

I would need to know the wheels dimensions and offset to tell you fitment calculations. Find some wheels and tell me.

 

GodSpeedGTA; That link didnt work. (took me to a discussion about broken lakewood traction bars.) The guys @ discount tire co. told me that they dont know the offset/size that works on these cars and different companies make different offsets/ sizes but if I could bring them the offset/size then he could run a search based on that info and let me choose from the results of that! I want a billet wheel, something that shows a lot of brake if you know what I mean.... The bigs/littles look is pretty cool though... I wish I could have seen your car. Got another link?

 

Wow that is retarded (the link) I straight copy pasted it. Well heres a quick pic and ill repost the link and check it.

Go into auto detailing and appearance, click on firebirds and then like 3 posts down youll see "artistic shots of firebirds" click that and go to page 7!

 

OK. Billet is going to be very expensive (typically $2k plus), however they can also make the wheels to any offset in most cases which is great because you wont need adapters then. I actually work for Discount tire (4 years) so you know youre talking to the right guy. Some really popular companies for billet wheels are boyd coddington, billet specialties, and boze wheels. I believe all of them can make the offset to your desire.

I need to know what you plan on going with in terms of tire width, wheel width, and I can help you search for wheels a bit if youd like just let me know exactly what styles you like. I know you want to see brakes, me too, do you like five spokes, multi spoke, muscle style or any more specific stylings?

 

Now those pics. were worth the wait!! That is one beautiful T/A! That is exactly the stance and fit I am looking for! I love the bodywork on your car. It just goes with the wheels so perfectly! I like to push the limits a bit so I believe I will probably go with the same sizes/offsets that you did.. I really like a deep lip like the ones on your T/A, So I would probably Like something in a five spoke. I have seen a five spoke that looks like the middle was cut out of each spoke. I dont know who makes it, but I like it. What happened to discount tire? I used to buy wheels from there all the time, and then a few years ago I went to buy some wheels and was told that the big stand of catalogs was not going to be carried in the store anymore... I was like wtf!! I too worked @ discount In tucson back in the late 80's and loved it! Too old to bust tires anymore... Anyways, Thank you! Any info you can give me on this is greatly appreciated!!! --Jason--

 

Well, I dont know who told you that because we definately still carry the catalogs and we order wheels and such.

http://www.tiresdelivered.com/produc...?cat=1041&pg=3

here is a design i think you are looking for. Not too bad on the price...way cheaper than billet wheels that run $2000 and they come in black or chrome. If you want to do the staggered 19 and 18 combo youll have to call that company and see if they are able to work with you so you can order two of each size that you want. I would go with the 19x10 for the rear and the 18x8.5 for the fronts.

I think if you got those wheels Id be kinda jealous too. When i was first looking for wheels i was interested in 5 spokes. Anyways, if you need anymore help or fitment guidance just reply or send me a pm.

 

Niche Venice. NIce!! I like them a lot! That is exactly what I am looking for. I like them in black. What does the offset mean when it says 30 or 40? I thought offset was measured in inches. What offset, using this system, fits my car? Yeah, I was really dissapointed a few years ago when I walked into that store and the guy said they didnt have the catalogs anymore. "Go online" he said. Do you think he was messing with me? They werent on the counter where they used to be. I was pissed! I had the money and was ready to buy!! Maybe its time to go in and see... Thank you! --Jason--

 

I think it might be that we carry a larger selection viewable online but every store should have catalogs. Anyhow, i think that wheels offsets are always measured in mm and then most of the other dimensions are in inches. Basically, wheel offset is where the center structure of the wheel or face of the wheel sits in relation to with rim cylinder.

My gfs dad also had some sweet 5 spoke tsw wheels on his chryst crossfire (19" rear and 18" fronts). Looked pretty cool.

 

The thumbnails were very informative! Thank you! I think I will take a trip to discount this weekend and see what has changed.... I noticed you are wanting a race built 700r4. I just had mine done. It has a full hughes master racing rebuild kit, Billet "b" shell, Hardened input shaft,corvette servos,shift kit, hp boost valve,Billet sun shell, Racing sprag, High volume pump, Transbrake, and about a hundred other parts that I dont know anything about..... The total for all of this including parts and labor was a little over 1,100 bucks, built by a shop that builds a lot of race trannys. I could get you the same deal...( I called around and got quotes from 2,000 to 3,000, so I know its a good deal...) Thank you!!! --Jason--

 

Thanks for the heads up! However, i think ive decided to go with a t56 swap eventually. I think it would be more fun to drive that way. Thanks again though if i ever decide to go the other way ill get in touch with you for sure. I need to see some pics of your ride. If you need anything else let me know, i cant promise i wont run out of knowledge though.

 

Well they are kinda ugly right now, but soon I'll send pics. Could you find the conversion for wheels? I still dont know what size fits. I mean what does 30 or 40 mean in The long run within the offsets you gave me?
Thanks!!! --Jason--

 

Alright heres the long and short of offsets

I base my calculations off of GTA crosslace wheels BUT all 82-92 models are the same structure and body wise so it shouldnt really matter.

Stock GTA wheels Offset:
fronts: 0mm
rears: +16mm

GTA wheels are 8" wide so in the front you have 4" of backspacing and 4" of frontspacing. Now, in the rear you have 4" + 16mm (or .63") which gives you a backspacing of 4.63" and a frontspacing of 3.37". (the positive offset forces the wheel deeper into the well basically tucking your wheels)

1 in = 25.4 mm

so lets give an example, say you go with the 18x8.5 and 19x10 that i suggested before and the 18s come in a +30mm offset and the 19 comes in a +40 mm offset.

first step: 8.5"/2 = 4.25" at the dead center of the wheel itself, however you have a 30mm offset as stated before so you need to calculate that to determine backspc and frontspc.

30mm/25.4 = 1.18" so your total backspacing is 4.25" + 1.18" = 5.43" and your frontspacing is just your total width minus you backspacing obviously. So its 8.5 - 5.43 = 3.07

So consider what stock figures are 4" front and back spacing so try to get close to that. If you add a 1" thick adapter then you push the wheel back out a bit and your new dimensions are:
Backspacing: 4.43" and frontspacing: 4.07" (because you just add 1" to your previous figures)

Now analyze your result. Your wheel/tire is going to basically appear in the stock location due to you having the same frontspacing as your stock wheel/tire does. You are simply "tucking" the rest of the wider new wheel. As long as you dont tuck too much you wont have any rubbing issues and the wheel will fit nicely and have clearance. At under .5" added to the backspacing in the front wheels, you wont have ANY issues!

Ill do the rears quicker but itll give you another example of how to calculate what your gonna get in advance. 19x10 so 10"/2 = 5" but it has a 40mm offset. So you calculate 40mm/25.4 = 1.575" so your total backspacing is 5 + 1.575 = 6.575"

NEW INFO: You typically (depending on the alignment of your rear end) have about 2" of safe tucking space. Remember, stock you can handle 4.63". So, you could possibly run the wheels on the rear without any spacers but it would be extremely close. Best option, would be to go ahead and use a 1" adapter. New backspacing 5.575" and front spacing 4.425". Youll have added some sweeet looking tucked wheels that wont rub in the back. Now remember to analyze the frontspacing too. Stock you were looking at 3.37" and now you have 4.425"

So your wheel/tire is going to stick out about 1" farther than stock but if you really look at it and use a ruler or the width of your finger, that isnt much at all and its not going to look like you need your name in old english on the back window. But you are putting 2" wider wheels on your car and wider tires so the inches have to go somewhere!

I hope this helps and let me clear up one last thing. Adapters are not the same as spacers!!! Adapters utilize new studs while spacers shorten your stud length and put more stress on your stock hub/stud assembly. Also, the reason why I always used 1" is because 1" is the smallest thickness you can buy. You can buy btw 1" and 2.5" or so thick adapters from ebay or adaptitusa.com (more expensive) Spacers are made as small as 1/4" thick but they are not safe and you could have a wheel come off easily.

So when you decide on which wheels give me all the info width, offset, diam. etc. and Ill help you decide/confirm (i hope you can try to do the calcs yourself) which adapters you need and what size tires you should run.

ttyl

 

Good God!!!! I always hated math!!! I think I understood that, though! I would still like to give you the measurements when I go to look at the wheels to confirm before I go and make an expensive mistake!! Thank you!!!

 

I think that would be too close for comfort. I tried some Crager Drag Lite rims on the rear of my car today. They have an 11" overall width, 10" bead to bead and 6.75" backspace...



I'm going to put 1" spacers on them and they should look/function pretty good (even though they are only for the track).

 

Might want to go 1.25" on those adapters for those draglights if you are gonna run fat tires JUST to be safe. I think my accuracy still stands, as I did say he would be better off with 1" adapter and 5.575" of backspacing. Not to say you are challenging what I said, but I think your example goes to further prove my calculations are correct. 6.75" of backspacing is just too much for our cars, and 5.5" or a tad more is just about as much as you can go without rubbing under turning, braking, etc.

Thanks for posting that example with pic that should help lots of ppl who keep asking the same sizing questions regarding offset.

 

Actually, thank you

I found this thread in searching for a solid answer, and just decided to try them out for fit and you see the results.

My comment was geared towards your comment earlier "might be able to get away with no spacers", and was just confirming your "new info".

I also noticed you help a lot of people with these kind of questions, good deal. I may not be a guru on fbod suspension, but come and visit the V6 forum on occasion :P

-john

 

Hello,

I really need some help here, I am doing a 502 swap, trnamission swap (auto to manual) AND ETC... I bought a new 12 bolt moser rear end with stock dimensions of 91 transam GTA. I will buy an enkei lightweight rim but I have no idea about offsets, backspacings, etc. I am thinking of buying these: http://enkei.com/RacingSeriesSpecs/RPM2.html

Can you just tell me the right dimensions? I came to US just to buy parts to my car and I am running out of time. Thank you for your help...

I have 12.2" wilwood disc brakes at front and rear...

 

I would PM GodSpeedGTA with these questions.... Though, I believe all of the questions you might ask are answered, very well here!!! This is a really good thread, if I do say so myself!! (And, I do!! LOL)

 

to GodSpeedGTA im looking at boss 338 wheels staggered for my TA, 18x8 and 18x9.5 what do you think the correct offset and backspacing would be for these rims.. would like them to look like your car on how they stick out! any help would be great! also tire sizes, im looking at getting BFG G-FORCE KDW2s in guessing on sizes of 255/35/r18 front and 295/35/r18.. is this to wide for rear?

 

Here is what I am thinking for my car. Can anyone please tell me if it will look goofy or not work. I want to get the Yearone billet cross lace wheels. 8.5x18 front zero offset, and 10x19 16mm off set rear. what it does is takes the stock zero offset front and 16mm offset rear and puts the added inches on both sides of the tire. Fronts will go an additional 1/4 in to the suspension and 1/4 in to the fender. The rear will go 1 full inch to the suspension and 1 full inch to the fender. I know it will work but will the rear stick out too much to where it looks stupid?? All your opinions are greatly appreciated. I have done so much re-search on this but before I spend 4 grand on wheels I want to know that it will look good.

 

fronts fine

rear will work, but i personally would like to see about .25 more backspacing. those wheels will have 6.129" of BS, which will work, but will be out a little too far for my liking. they will be about flush with the quarter panel opening lip. It wont look stupid some some people here with wheels sticking 2" out past the fenders. I just like them tucked a little more.

 

Thank you 86TA. My intentions is to have the biggest lip/ deep dish I can but still look good with the stock suspension. Usint the stock off set in the rear with the bigger tires gives me 5.6" backspacing and a lip of 4.4" I agree that will take the flush to the quarter panel. i wish I could have a well to test fit or a picture of one set up like that to see if I like it.

 

maybe I will go with this. What do you think?

Current Wheel Width 8 Inches, Current Wheel Offset (16 mm) New Wheel Width 10 Inches New Wheel Offset (25 mm) Wheel Edge Movement ChangesSuspension Side 1.4 inches Change Inches Fender Side .6 Inches

 

thats 6.5" of bs, it will tuck like a stock "front" 16x8 does on the rear. It may require some inner wheel well work, hammering, maybe some cutting of the bump stop area. If you are ok with that.

 

Thank you 86TA!

 

Just wondering if a brake upgrade might throw the fitment off. I am also thinking about wheel sizes, and would also like to do a brake upgrade.

 

i want to run 15" rims 8"wide with 5" of back spacing in the front, and 10" wide, with 5" back spacing in the back, will that work, and will they still fit like a normal wheel in the wheel well? (not sticking out)

 

No. Front will need a littlebless backspacing and the rear needs at least 6 to fit the way you want but that will not work on the car without some sheetmetal work to the inner fender. 15x8 with 4.5" or bs will fit all around nicely without any drama.

 

 

I don't know if you're asking if that will fit or not (it won't without a roughly 2" spacer), but be aware that some of what is posted in this thread is not correct, especially in the response that has all the math in it.

The deal is that the backspacing (bs) and front spacing (which is rarely talked about but can be important to get just the stance you want) is measured from the bolt flange to the lip of the wheel, while the wheel width is measured from bead seat to bead seat. Offset is how much the hub mounting pad is offset from the wheel barrel centerline. Depending on the wheel the lips might be 3/8-1/2" adding that much to the spacing over what you would expect from the published wheel width, for example, an 8" wheel with 1/2" lips will measure 9" wide and a BS of 4.5" will make it 0 offset.

The stock wheel fitments are:
15x7 with 4" backspacing (roughly 1/2" offset)
16x8 with 0 offset on the front (roughly 4.5" backspacing) and 16mm (~5/8") in the back (roughly 5-1/8 backspacing)

I know that I went from backspacing to offset there, the factory spec changed.

With wide tires what works best is something close to the 16" wheel spec. Some people will go 0 offset on the rear but that results in tires that stick out a little. Everything MIGHT rub on the front, factory wheel/tire combinations rubbed on the front with some alignment settings. Cars with original 15" wheels came with thinner bump stops in the steering box allowing you to turn the wheels further so a narrower tire will rub than a car originally equipped with 16" wheels, but they will rub with the tires turned the same amount of degrees- realistically, you shouldn't be turning the wheel to the stops anyway, that causes the power steering pump to overload and bypass (the high pitched sound you get from turning your wheels all the way).

For front skinnies, well most people are trying to either tuck them further so they can run a taller tire or otherwise you're limited by the clearance you need for the front brakes and typically there aren't many offsets available, it's usually one will work and the other won't

Finally, if you want to get really **** retentive about this, tires measure a specific width on a specific rim, if you dig a lot you'll find those specs. Changing the rim width 1/2" changes the tire bulge about .2". With that information, you can calculate exactly where your rim lip and your tire bulge will end up vs the stock wheels/tires and fender lines (years ago I got tired of doing the math every time and made a spreadsheet).

 

Oh, and one more thing to look out for, on the front, too much BS will run into the tie rod end. How much depends on the thickness of the wheel material, it's shape, and the tie rod ends on the car (some are chunkier than others). You need to start measuring over 8.5 or 9" in width, and the bigger the wheel diameter the more room you'll have here (I think some 19's will fit around it completely, but I hate big diameter wheels so I've never figured that one out exactly). Back in the 80's and 90's there were a few wheel manufacturers that made special rims with thin inside lips just to clear more BS on the front of these cars with a 16" wheel/tire.