I wanted to share some information I came across in my research...

We all know that going to wider tires helps out the cornering and straight line traction of the third gens as well as almost any other vehicle. Obviously this is the case as the Z28 came with 215-65-15 tires, the IROC/TA 245-50-16, and the 4th Gen SS/WS6 with 275-40-17 rubber.

The question is this - what is the proper sidewall profile and wheel width for a given situation? I'm going to expound on what my research has found out...

Wheel width came to me when researching wheels and tires for my girlfriend's E46 BMW. BMW did something similar to GM when it came to performance levels and rubber, there were tires in the 15", 16", and 17" diameters offered, some even in 18"; but the real story is the wheel width for each tire. BMW has a 185-65-15 tire on a 15" x 6.5" wheel; a 205-55-16 on a 16" x 7" wheel, and a 225-45-17 on a 17"x 8" wide wheel. BMW has put a 225 width tire on a 8" wheel that GM put a 245 width tire on for the third gens. The reason why is the lessen the tire's overhang of the wheel which in turn stabilizes the side wall and actually helps both handling and steering feel. It seems the formula they use is tires section width minus 1" is their wheel width - example being is a 225 tire has a 9" section and is put on a 8" wheel. With that rationale, the 245 tire that GM put on an 8" wheel needs at least an 8.5" wide wheel (if not 9") to stabilize the sidewall. And a 275-40-17 tire needs a 10" wide wheel, not the 9" wide one GM used on some of the 4th gens.

Tire profile is another interesting subject as it depends on both the tire's overall diameter and the suspension it is used on. Obviously GM cheaped out on a few levels when it comes to the third gen suspension - using pocket springs instead of coil overs in the front and going with a solid axle in the rear. From what I can gather, once you go larger than a 17" wheel (with a stock sized tire diameter), the car will have a hard time keeping the tire patch on the road with stock style suspension. But if you convert to front coil overs you can run about 1/2 the spring rate and get the same results, which will allow for a 18" wheel (to clear larger brakes) and give less sidewall flex) with good results. The problem is that the coilvers cut down on clearance, probably giving a max tire width of 255mm in the front. Out in the rear, you really cant do much, but there is an easy work around. a 17" rear tire will have a 8.75" sidewall at stock height, an 18" will have only a 7.75" sidewall (danger area), but you can run a 1" taller tire in the rear (like the C5 Y bodies) to make it more reasonable. Of course the 4% increase in diameter will screw with your speedo, but it can be rectified with trans speedo gears.

Comments and discussion welcome...

ok, I have a 1985 I-roc. Im trying to buy some up to date wheels. What I was trying to do was 17" in the front and 18" in the rear. The problem im running onto was the offset on the wheels. What would you suggest to fit this application best. I hope you can help me.

an idea of what width wheels you are looking for would help. as well as a style

Bingo...

A set of 275-40-17 on a 17" x 9.5" wheel (factory ZR1 for example) is an easy way to get some inexpensive wide rubber on your ride.

Yes & no....ZR1 wheels (any 4th gen or C3+ Corvette) require a spacer to fit.

I'm a fan of direct fit wheels, like my 17" x 9" ROH Snypers, 0" offset/5" bs. 275s front/285 rear, no adapters.

im not a fan of spacers either, they may be perfectly fine, but i just dont like them.

I'm weary of spacers also - 3,500lb cars pulling 1.0g or more can prove a weakness real quick.