I'm going with new wheels on my Formula (18x8.5 40mm offset fronts and 19x9.5 45mm offset rears), and I know that going with a 35 profile tire on the fronts will be appropriate, my question is what profile for the rears?

I think being a 19" wheel they need to be a 30 profile to not throw off the speedo, but I just wanted to make sure.

 

In the back, to keep the speedo correct, you need to match the stock tire. What size came on your car from the factory - 245/50-16 (25.7") or 235/55-16 (26.2")?

Diameter is a function of profile and width.

To match the diameter in the front, you would be looking at a 285/35-18 (25.9"), but that tire is too wide for an 8.5" wide wheel. You need at least a 9.5" wide wheel for the 285. For the 8.5" wide wheel you could use a 255/40-18 (26.03")

In the back you could use a 285/30-19. Some manufacturers call this a 25.8" tire, others a 26.0" tire.

 

Well, the car has the WS-6 factory 17 inch wheels on it, I would have to check.

I do know that on my LT1 that I have 18x9 wheels on the front and 275/35ZR18's in the front (perhaps they are the wrong size). Then on the rear 18x9.5 I have 305/35ZR18's and the speedo seems correct.

Thanks for chiming in. Sounds like a 30 profile for the rear should do it as I suspected.

 

The WS6 coupes had 275/40-17 (25.7")

275/35-18 = 25.6". Again, minimum wheel width for that tire is 9.0". But you could get away with it on a 8.5" wide wheel. Might be a bit scrunched, but barely noticeable.

305/35-18 = 26.4". Requires a wheel at least 10.0" wide.

The speedo error between a 26.4" tire and the stock 25.7" tire would be the speedo reading 2.8% slower than actual speed, or for example 1.7MPH lower than actual speed at 60MPH.

 

You are correct on the size Fred (I just checked the Formula).

The Nitto 305 seems to look fine on the wheel, but I think I mispoke earlier and its actually a 10.0 wide wheel in the rear (and Nittos have been known to be more narrow than other brands).