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My car does have the sport diff, and the Audi select drive. (S-Line comp pack) What I have found out is that I will need to change the front and the rears to match the overall tire diameter. It looks like a 245/40/19 and either a 275/35/19 or a 285/35/19 will be within 4 revs per mile difference and either 1/10th or 2/10ths of an inch in overall height.
The SD is quite distinctive in appearance from a regular differential.
I have been running a 2015 S5 Technik with 255/35/19 et32 and 275/30/19 et32 in the summer for 50K kms and it is as good as the day I bought the car. After a few hard corners with understeer over the starting months the diameters for front to rear were almost identical. The sport diff is more robust than people think ,, at least I'm my case. The slightly wider rears is much more pleasing to the eye.
It looks like different diameter tires front/back would cause excessive wear on the center differential's planetary gear and the clutch plates. When everything's perfect, with 40/60 torque split, the small planetary gears don't spin (at 3:10) When torque requirements change due to slippage or different diameter tires, the little planetary gears kick in. I guess that's why Audi's so emphatic on using tires with same threads and diameters on all four tires.
The sport differential is in the back, so it's not affected by different front/back tires.
Have you ever drained and filled the center differential fluid? How did it look? Mine looked black after just around 60K miles when I drained/filled it. And I had on stock wheels.
I'd keep the tires the same all around. BUT, keep in mind that if you put on larger diameter tires, your speedometer will read slower. You'll need a VAGCOM tool to adjust the tire size to make it more accurate!
04-26-2015, 06:44 PM
get under the rear and take a pic.
