I've heard that having tyres with big deviation in diameter from the stock (+/- 2.5%) is detrimental to the drive train and also the speed is shown wrong.

997 Turbo comes with 235/35/19 tyres in the front.

On 9x20 rims that are on the car the 235/30/20 tyres look too narrow, the rim sticks out. The difference in diameter is only 0.31% though.

Would it still be OK to put 245/30/20 tyres on the front, even though the diameter difference is now 1.24% ?

What are the negative effects of these tyres on the car?

This is a very important and serious question for me!

Thanks in advance!

P.S. Speedart BTR uses 245/30/20 tyres up front:
http://www.sportauto.de/bilder/speedart-btr-ii-580-evo-im-tuning-test-der-porsche-911-als-bi-turbo-racer-3891385.html?fotoshow_item=5#fotoshow_item=4

 

Peskarik

What tires do you have on the rear? Which deviation do you have there?

The eventual trouble lies in the front/rear ratio deviation. Not in the front itself, not in the rear itself.

 

Rears sit on 12x20s and are 325/25/20.

Deviation to stock 305/30/19 is 0.62%.

Basically, if I put 245/30/20 on the front that will be exactly like on SpeedArt BTR II 580 EVO, but I am not sure if one has to adjust something somewhere (transmission, speedometer) in order to be able to fit these tyres.

 

I won't make the calculation there but if you have the same trends front and rear (for instance, let's say + 1% front and rear), you won't have any trouble.

as a reminder, calculation example for a 325/25x20" tire.
Diameter = 325x0.25x2+25.4x20

 

from tyre calculator
245/30/20 = 656mm
235/35/19 = 648mm
+8mm = 1.23%

325/25/20 = 670mm
305/30/19 = 666mm
+4mm = 0.62%

 

oem 19" ratio front / rear = 638.875 / 656.45 = 0.9732

20" ratio front / rear = 655 / 670.5 = 0.9768

It looks quite similar to me.