I have done my research on this and I just need some clarification and opinions. I’ve got 225 40 18 on the front and 255 35 18 on the rear of a quattro car. Everything I’ve heard about this usually says any variation will make my diff explode and the world to end. Running those numbers through a tire size calculator yields a 0.24% diameter difference with an extra 2 revolutions per mile.
Circumference: 78.79 in 2001.2 mm
Circumference: 78.60 in 1996.4 mm
Revs per Mile: 829.3
Revs per Mile: 831.3
Actual Speed: 60 mph 100 km/h
Speedometer1: 60.1 mph 100. km/h
Speedometer Difference: - Speedometer Difference: 0.241% too fast
Diameter Difference: - Diameter Difference: 0.24%
this is from tire rack:
Audi - As published in their vehicle owner’s manual, “rolling radius of all 4 tires must remain the same” or within 4/32-inch of each other in remaining tread depth.
(4/32) in = 3.17500 millimeters
My difference (4.8mm) - 3.2mm = 1.6mm
The extra 1.6 mm outside of factory tolerance doesn’t sound like it would cause a problem. I’m assuming factory tolerance is slightly lower than what is safe as well to protect Audi from a legal standpoint…
I plan on getting 225 40s for the rear anyway, but I have run into some other expenses so I am putting it off for a bit.
My question is is this difference negligible and can I compensate the difference by running slightly lower pressure in the front?