I just looked at my tires, and it's VERY odd. The back tire appears bigger, but it is a 205/55/16, while the front tire that appears smaller is a 205/60/16. YET get this, tirerack.com says the original equiptment for my car is a 245/50/16. So obviously, this confuses me a LOT.

Can someone straighten out the tire deal for me? I might be buying new tires soon.

Is it even possible for a 205 to fit on something that was originally a 245? My mind is like .

 

Bump. Could someone set this tire deal straight? It has me confused, and I've never really paid attention to tires.

 

If the wheels are 16x8 then yeah, probably would have had the 245/50/16 tires. Apparently 205's can fit on the same wheel if that's what's on yours.

 

Were there ever any different size rims offered on Irocs, besides the 16x8's? I don't see how it'd be practical for a 205 to fit on a rim that's meant for 245's, just seems like too much of a difference.

I think when the time comes for me to buy tires, I'll just have the original 245/50/16's thrown on and see how it works out.

I did come across something interesting though, I went to 1010tires.com and used their tire size calculator, and figure out the profile of it isn't a problem at all.

245.50.16's -

Section Width = 9.64 IN. 245 mm.

Overall Diameter = 25.64 IN. 651.25 mm.

---------------------------------------

205.60.16's. -

Section Width = 8.07 IN 205 mm.

Overall Diameter = 25.68 IN. 652.77 mm.

---------------------------------------

Now I understand 4/100th's of an inch difference in tire profile will barely affect anything, except maybe a couple revolutions per mile, but what I'm worried about is width.

Is it safe at all to put a 9.64 inch wide tire, on an 8 inch wide rim? The 205 seems like it would be the original and correct size, by being 8.07 inches wide, for a 8 inch wide rim.

If anyone that has more knowledge about tires than I, PLEASE chime in, this is bugging me a lot. I'd really like to know for a fact if that's even possible to put a tire on that's almost 2 inches wider than the rim it's self.

 

Someone wanted to save money and installed smaller & in most cases non-speed rated tires. IROC's with the stock 16x8 wheels work best with
245/50/16ZR directional tires.

 

Thanks, I've been looking for a straight forward answer to that for a while, I noticed the difference sizes before.

Guess the last owner was a tad on the cheap side.

 

Also note: The rear wheels must stay on the rear, and the frnts on the front. The rims say front and rear casted into them, and they cannot be swapped - make sure the tire changer knows this.

 

Here's a pic of my car with 245/50/16 on the front and 255/50/16 in the rear just to give you an idea.

 

Can you elaborate on this please? I've just got a set of stock 16"s for my car.

 

The wheels are marked (front) & ( rear) which means that they are axle speciffic. This also means you CAN NOT rotate your wheels & tires front to back. If you want to rotate the tires they will require dismounting from the wheels.
If by chance you bought IROC wheels which are not a matched set (front & rear) in my opinion you are SOL.

 

I understood that much. Why can't I rotate them back to front?

 

Big difference in the front wheel offset. Take a look at your wheels marked front and compare them to the back. the difference will jump out at you.

 

Does this affect the 15" too?

 

no, 15" stock wheels are the same on all 4 corners

 

So why are the 16's different? Is the back wider for some reason?

 

The 3rd gen platform was designed originally for steel wheels which have thinner material for brake caliper clearance. When GM fitted them with the sportier IROC wheels, they discovered 2 issues-
1) The aluminum front wheel design would not fit over the factory brakes with the same offset as the steel wheels. They had to move the offset 16mm on the front for clearance changing the scrub radius as well to more positive which is bad(a somewhat inherent defect- that can be corrected through knowledge and inginuity)
2) did not allow the rear to have the same track width needed for good lateral balance when using the same 16mm offset on the rear with the wider 16" wheels sports car package going against the competition. the outside edge of the wheels must stay in parallel track width for ultimate lateral handling and scrub path- so they opted to make two different offset wheels for the chassis in a bandaid fix for wide tire sport use. Hence the 'fronts' and the 'rears'

I personally GM engineers were struggling to learn stuff in those design years because some cars were even fitted with small rear spacers from the factory based on different hub offsets of different brake packages upon release because they were correcting handling flaws in balance through changing track widths front and rear based on factory wheel and tire options.

Dean

 

Food for thought for everyone fitting wheels and tires on their cars-

Whne you fit differnet wheels and tires and you are changing the track width, even if the tire is one inch wider on both the outside AND the inside of track width (example- the front track lets say is 70" width at the outside tread footprint of an 8" wide contact patch/ and the rear is 72" to the outside tread footprint of a 10" wide contact patch= the rear is 1" outer track than the fronts on each side= positive rear track will make a car slightly ight on corner entry under braking, slightly loose at steady state cornering, then way loose on exit under throttle.

A negative rear offset will do a little affect in reverse fashsion but not a complete visa versa- this is still a bandaid fix in handdling. you wnat the tirew patches in line with each other front to rear. They need to follow the same path through a corner when it comes to the outer edge.

 

Wow, I really appreciate the explanation, but I'm struggling to comprehend this. Can you explain it in more layman's terms?

 

Backspacing...

The point at which the rear wheel is attached to the rear axle is farther away from the axle than the same attachment point on the front wheel. So when a rear wheel is put on a front axle, the tire is deeper inside the well, and thus will rub on the suspension (ie tie rods ends and such that are not present in the rear well).

Bottom line is, 16" wheels marked "rear" in the castings will not work on the front end of the car as they will rub, and thus rears can only be put on the rears.

 

I see. So from a performance stand point, am I better off with sticky tires on a set of 15's?

 

Its this simple, go out and buy some very good quality 245/50-16 tires.

That's the best performance upgrade you can get for your money "Dollar per performance gain"

 

Bottom line first - take Vetruck's advice and get a set of good 245/50's.

About your concerns - be careful about using any of the online "tire calculators". They don't always tell you everything that's going on. In this case, tire section width (the 205 or 245 number here, sort of) varies with the width of the wheel it's mounted on. A 205-width tire is about 8.1" wide when mounted on it's "measuring width wheel" that's 6" wide. On an 8" wide wheel, it will "grow" to about 8.9". That's a mild "stretch", being slightly above mfr-recommendations of 7.5" max. But in my opinion it is still do-able in most cases (and much preferable to being half an inch under the mfr-recommended minimum by trying to run them on 5" wide wheels).

In a little more detail, tires have about a 1.5" to 2" range of wheel widths that they can be fit to per mfr (Tire & Rim Association, actually) standards, with the measuring width wheel falling at or near the middle of this range. IIRC, for 55 and taller profile tires, the "measuring width" is about 70% of the nominal tire section width. So yes, it's entirely possible to end up with tires approaching 10" wide being legitimately fitted to wheels that are 8" or even only 7.5" wide. FWIW, 2" wider is only 1" of "sidewall bulge", and almost half of that is "hidden" behind the wheel flange, so it won't even look odd.

Don't try to work too precisely with tire nominal dimensions - tires are permitted to vary somewhat from what the advertised size indicates. I think there can be as much as 7% variation in dimensions between two tires that claim to be "exactly" the same size as molded into the sidewall. Use tables provided along with some of the online tire catalogs. Here's one that happens to have both of your 205-width sizes in it.


One last edit - neither of those 205 tires are really adequate for a car of this size, weight, and performance intent, based on load capacity.


Norm

 

It may be to late, but you could write GM and tell the Engr. Guys that they put the wrong tire on 16x8 wheels. The Factory Stock 245/50/16VR tire was too big for that 8" wide Wheel per your calulations.

 

That 70% figure came from somebody who works within the tire industry.

Specifically, it's included inside a calculation for estimating tire load capacity at any given inflation pressure. Normally, you'd assume that this would be to determine the tire's allowable load under its design basis condition, including wheel width (and I'd seen the 70% figure elsewhere but don't remember where).

It does seem to match up a little better with minimum recommended width, so perhaps that formula is intended to reflect the situation in a minimum recommended application rather than design basis. To that extent, I stand corrected.
<makes note to self>


Personally, I probably wouldn't put anything wider than a 225 on 8" wide, or mount 245's on less than 8.5", given unrestricted choice in the matter.


Norm

 

Here's a good site I have bookmarked-

http://www.rims-n-tires.com/info_spe...20/%20Comparer

 

Better than most, given the pictures that adjust to your inputs and the frontside/backspace results.

However, it ignores the fact that the tire section width gets wider as the wheel it's mounted on is widened. Every half inch of rim width is worth about 0.2" in the actual section width. There are situations where this can be a deal-breaker.

And the revs per mile ignore tread compression (about 3.5% more revs actually occur than what that sheet and every other one I've seen calculate). For the curious, tire revs per mile isn't equal to what you'd calculate from the tire's radius under load either (it's somewhere in between).


Norm

 

I've always wondered if a tire expands in diameter at high speeds like 150mph, and at what speed they are true to size due to centrifugal force. I know again it depends on the tire, but I imagine the is a varying speed that all tires will eventually grow to size and then some taking away the weight compression of stagnant or low speed driving.

I have always thought this from a simple drag slick on a top fuel car but I know those are biplias and not radial. I would think even steel bands on a radial would eventually stretch or am I wrong?

 

Steel belts, like anything else, will stretch under load. Just not very much (how much being somewhat dependent on the specific details of the steel belting and other tire features).

A long time ago I was involved with time-speed-distance sports car rallying (up to and including the development of a section of an SCCA National Rallye). That sport is all about Precision with a capital "P". Steel belt radials were the only way to go (this being back when polyglass belted and even unbelted bias ply tires were readily available). But even SBR's are a tiny bit sensitive to speed with respect to how many revolutions per mile they give, so it was important to drive the odometer check leg at close to the same speeds noted in the route instructions. Screwing up your odometer calibration could easily end up costing you finishing positions, since a good team could frequently drive legs to within 0.02 minute of "official" time. That small amount of "error" is about 0.1% (roughly comparable to a difference of one tire revolution per mile).


Norm

 

Install these on your car the technical problems of correct tire width will be solved.