Dirk Miller

Wonder if anyone else has had the same issue when trying to use a different sized tire?
I have been buying “semi used” tires from on outfit that obtains their tires from California auto dealerships when new sets must be installed to qualify a car for dealer’s OEM certification. So when I found a deal on a new pair of Potenza RA for the rear of my ’08 C6, I bought them. They are 285-40-19. I had been using Goodyear Eagles 285-35-19 while my fronts are Potenza RA 255-35-18. The used tire dealer had pointed out the diameter was slightly bigger on the new set, but thought they should work. After installing the new rears, I discovered my brakes would not engage properly under hard braking. I’m assuming that my TPMS must be catching the difference in the circumference of the front and rears; so I had my old 19” eagles reinstalled. The braking is once again fine. In thinking about this, I measured the circumferences of the tires and found the following: fronts = 77.50”, rear eagles = 82.50” and new set were 85.25”. I am again assuming the delta on circumference #'s has caused the computer to not accept.
Any one else experience something similar? (oh by the way, I am aware that I have run-flats on rear and non run-flats on front).

c5racr1

Anything more then 1 inch hight difference will cause all kinds of traction control issues. Only thing you can do is get the tires within an inch.

carefulnow

Don't plenty of people run much larger tire diameter differences on C6s without issues?

Example: https://www.corvetteforum.com/forums...t-oem-c6z.html

Turn off traction control?

Bill Dearborn

First, the TPMS has nothing to do with the issue. All it does is broadcast an ID code and a tire pressure reading. The wheel speed sensors send wheel speeds to the EBCM (electronic brake control module) which sense speeds that are outside the normal limits and either apply the rear brakes (traction control) to stop the rear wheels from spinning too fast (rear tires too small in relationship to fronts) or will try to add throttle to speed up the rear tires (engine drag control) if the rears are too large in relationship to the fronts. There is no such thing as a computer accepting or rejecting a tire. The EBCM looks at wheel speeds of all 4 tires and determines what to do if they are different than what is expected for the speed of the car, the lateral G's being experienced and the amount of steering wheel angle.

In your case the front tire is about .7 inches smaller than stock(245/40/18) and the rear tire is .8 inch larger than stock (285/35/19). This combo made the front to rear ratio even worse. If you had a smaller tire on the front you had to use a smaller tire in the rear. The stock ratio of front to rear diameters is 1.04 (rear diameter/front diameter). Your ratio is 1.118. Either get smaller rear tires or larger front tires. Check the diameter of the tires you are considering and then use that to calculate your ratio if you purchase them.

Mixing brands and types of tires from front to rear doesn't make a big difference as long as you have the best handling tire with the most grip in the rear. If it is in the front you stand a chance of looping the car.

Bill

c5racr1

The tires have to be 1.00 inch relative to front and rear. You could have 26/27 with no issues. If you have 26/27.5 problems because the ratio is more then 1.00

naschmitz

Maybe too much rake is a bad thing but I run Hoosier Racing Radials that have only 0.1" diameter rake front to rear without any braking problems on the track (2006 C6).

Stock Goodyears were 25.7 and 26.9 inches in diameter; Hoosier Racing Radials are 25.5 and 25.6 inches in diameter.

Dirk Miller

The first line below are my street tires which also work on the track, even with a 2 1/8” delta. Go figure.

Stock tires: Fronts 255-35-18 -- diameter = 24.00”
Rears 285-35-19 -- diameter = 26.12”
2.1/8” difference

These tires work on the track for me.
Hoosiers : Fronts 285-645-18 -- diameter = 25.50”
width= 10.5”
Rears 305-645-18 -- diameter = 25.75”
width= 12.00”