First, don't use a calculator to determine tire diameter. Use the manufacturer's specifications which can be found on the Tire Rack Web site. Some manufacturers don't exactly follow the so called rules when it comes to aspect ratio/diameter.

I look at ratio Vs percentage. I divide the rear diameter by the front diameter to get the ratio. The C7 uses one EBCM for all models and there is some variation in ratio across all models. Your car came stock with a 1.0389 ratio. The Z51 version of your car comes with a 1.0271 ratio and the wide body cars come with a 1.0310 ratio.

======= Front ==== Rear ==== Ratio
Base === 25.7 ==== 26.7 ==== 1.0389
Z51 === 25.8 ==== 26.5 ==== 1.0271
Z06/GS = 25.8 ==== 26.6 ==== 1.0310

Bill

 

 

Some other things to consider. Theoretically, you can get by with a change of no more than 4%. To me that means you could go from a 1.0 ratio to a 1.07 without seeing much impact while doing normal driving. The issue usually comes up when pushing the car hard or in some sort of emergency or low traction situation. ABS works based on incipient lock up when one or more wheel is turning slower than others. If the ratio goes too high you could get into a situation where ABS activates early because it sees the rear wheels turning too slowly and thus releases brake pressure to the rear calipers. TC also works based on the differences in front and rear wheel speeds. If the ratio is too small then you could see early activation of TC under certain conditions. Stability control is initiated when steering wheel, yaw, lateral G sensor inputs are processed by the EBCM and it determines the car isn't yawing the way it should for the amount of steering wheel input. Then it decides to reduce torque for a short period of time or apply the brake at a single or multiple wheels to get the car to yaw properly in relation to the steering input. Inboard front brake application will get the car to turn in better to correct an understeer situation; outboard rear brake application to reduce an oversteer situation by keeping the rear from stepping out to far.

Torque reduction is the primary reaction in power on situations as it works faster than applying a brake. However, in off throttle conditions brake application is the only way to bring yaw under control. That is when the wheel speeds are used since the EBCM determines how much force to apply to the brake caliper and if the wheel is turning slower/faster because of a tire ratio change it might miscalculate how much force to apply.

To me that means you really don't want to be outside of a 1.02-1.06 ratio. Most people most of the time won't notice much difference even when going with big ratio changes. It is only when you get into that really hairy situation that you might not get what you need when you need it.

One thing I do know. You don't want the ratio to be less than one. I had a 97 C5 that I swapped the front and rear wheels on to see what would happen with TC. With the engine on, the transmission in neutral and the clutch pushed in the car would not coast down hill if Traction Control wasn't turned off. As soon as the brake pedal released and the car started to move the EBCM detected the rear wheels were turning faster than the fronts and applied the rear brakes in order to get the rear tire speed slowed down to what it should be. Basically, by swapping the wheels I turned the standard 1.04 ratio into a .96 ratio and the system didn't like that at all.

Bill

 

Ok.. better late than never... Here is some choices I'm looking at. Rims are 19" all around. Fronts are 19x9 and rears are 19x10.5.

All else being equal.. I mean some of these aren't apples and oranges... Hoosiers for example.. Any of these ratios you would stay away from and why? 1.011 too low? Or as long as rear tire is just a smidge bigger we're all ok....? Or is it that as long as front and rear are at least equal in size OR front leaning smaller?

 

I would try to stay below 27". Why not 305/30/19 for NT01s? They work very well.

 

 

 

none.

 

That's what I thought too but most are not seeing as good results with the r888r as the nittos.