NTMD8R

We know the front and rear tires have a diameter difference.

I have "heard" that the difference in diameter is monitored (via revolutions) by the computer,
AND,
If that difference is NOT within certain minimum/maximum,
it will put the vehicle into "limp" mode.

But I have never actually heard of any person who has experienced this.

I would like a definitive Yes or No that this does happen.
And, if so, what are those minimum and maximum values.

Thanks

Vanover

My tires are not the same on my C5 I don't have any problems

lt4obsesses

Not really sure what the exact question is, but I imagine that as traction control systems become more sophisticated and tolerances become tighter and tighter, getting too far off would throw the TC into panic mode.

If the rear wheels are too much taller than the fronts, the wheel speed sensors would make the ECM think that the front wheels were losing traction, which would make the TC apply braking, reduce fuel and torque, and whatever else.

professorjim

But what if they are the same size and the stagger is removed. What then is the impact on the ECU/ABS controllers?

dr_gallup

My 350Z also had traction control, stability control and staggered tire sizes so while it's a different car, it had comparable systems. I had 2 sets of wheels and tires, summer and winter. Once while swapping them I accidentally put the fronts on the back and vice versa. As soon as I tried to drive it like that the ECU thought that it was getting a lot of wheel spin (rears were turning faster than the fronts) so it pulled most of the throttle out. I was barely able to limp around the block and fix my bonehead mistake.

professorjim

I can see why. If the ecu was set for a 1" difference front to rear, and then it saw a direct opposite it would think it was a 2 inch difference. I was more concerned with if it went from a 1" difference to equal. I'm getting a lot of feedback that says going equal size would not mess up the ECU and would give added benefits of equal footprint, easier to balance the car, and rotational ability to reduce costs. All sound arguments IMO.
Thanks for sharing your experience.
professorjim

n8dogg

I think the issue lies within the overall diameter. You can run any wheel size you want as long as the tire diameter is close to the original. Too far off from front to rear and the car will see a too high of difference between wheel speeds causing ABS or traction control to come on and eventually set a trouble code. If the tire sizes are square, the only risk that I know of is a speedometer that reads too high or too low.

Bill Dearborn

The brake control module uses the wheel speeds from the wheel speed sensors to determine how to modulate the brakes for the ABS and Stability Control. The difference in front to rear wheel speeds is used to determine when to activate traction control. If the tire diameters change outside of a certain range the ABS activation will be affected. You may get a sooner activation of ABS and the amount of brake applied may be inappropriate. The same goes for the Stability Control. It isn't activated by differences in wheel speeds but it does apply the brake at one wheel or a combination of wheels to control yaw. If the wheel speeds are screwed up by tire diameters that are too far out of range then the force applied to control yaw can be wrong. With TC make the diameters equal and the TC may not activate while driving down the road but when the rear tires start to slip it will activate sooner and harder since it will be seeing more slippage. Not sure on the C7 but the C6 had Engine Drag Control and if you put too large a diameter tire on the rear you could get the engine adding throttle when you took your foot off the gas because the brake control module saw the lower rear wheel rpm as an indication the tires were slipping on a low grip surface and ordered the engine to add torque to prevent spinning the car.

The best thing to do is to keep the front to rear diameter ratio within 4% of the stock ratio so you stay out of the grey areas.

The car won't go into limp mode but it will perform differently than you may want. This is something that can't be tuned out by some turkey with a tuning program. They don't have access to the programming in the brake control module.

Bill