Hello. Can anyone tell me if my rear tires are 305x30x19 and I want to put 18" wheels on the rear only and put 305x35x18 would the car "feel" ok? I mean I would not brake the diff? it is 997.1tt car.

acording to my calculations the dire diamter difference is like only 4mm bigger, but when I put two tires side by side it seems more like 0.7" of a difference or so. is it ok to drive like this? if it is used only for drag racing 1/4 mile runs? no city or highway driving? would the diff be ok? what kind of diamter difference is permited and what is above what the diff can handle?

 

Those tire sizes match up pretty close according to Tire Size Calculator at http://www.tiresizecalculator.info/

The front diff on tt cars is calculated for the 235/35/19 & 305/30/19 tires sizes. You shouldn't vary the percentage difference between the front and rears by more than 2-3% or you risk confusing PSM which can cause some odd effects in driveability.

Always check the measurements of the actual tires you plan to use because not all tires of the same stated dimensions have the exact same rolling diameter or revs per mile.

 

Yes acording to calculations it is only 4mm difference. but when I mesure 2 wheels side by side it is like 20mm difference. 20mm from 670mm overall diamter is 3%.
is it ok to drive like this. only 1/4 mile runs. 10-15 of them. not highway driving or anything. is the diff be ok?

 

At 2-3% difference I don't think you have to worry about harming the diff but why not just disconnect the front diff and then you won't have any worries about what tire you run on the back.

You might be able to just pull the fuse for PSM and eliminate any problem there.

The real problem is with the PSM system thinking a rear wheel is slipping because it is rotating faster than the front wheel and then it kicks in traction control...and your run is ruined.

Having said all that I still think you should be OK with that tire if your front is still 235/35/19. The 305/35/18 matches up pretty close to 305/30/19.

Check the revs per mile on both tires. Tirerack.com can supply most specs on a great many tires.

 

i need awd for drag