rolling diameter of 235/35 tire on 9.5" rim same as on a 8.5" rim?

[AvanTTix]

quick question guys...

is the rolling diameter of a 235/35 tire mounted on a 9.5" rim the same as if mounted on a 8.5" rim?

i need to know ASAP...

also as far as the differential goes for Quattro is running the following combo okay:

front: 19x8.5" - 235/35
rear: 19x9.5" - 235/35 or 245/35

this is what i get from the Miata Tire Size Calculator:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/35-19 3.2in 12.7in 25.5in 80.0in 792 0.0%
245/35-19 3.4in 12.9in 25.8in 80.9in 783 1.1%

[S14-8E]

Are you getting another set of rims?
No more Sportec?

[T-dot B6]

8.5" and 9.5" has to do with the width of the tire, so im pretty sure that has nothing to do with the diameter of the tire. As far as running your tire combo, i would not recommend going with 235 in the front and 245 in the rear. This is because you will have different rolling diameters. On awd cars like ours it will mess up the differential because in our case quattro will think two (or however many) of the wheels are slipping, and therefore will try to correct it.

[F16HTON]

I measured mine with a string and the differnce is completley negligible.

The 245 vs the 235 mathematically will have about a 1% difference.

The quattro is cool with up to 5%

[A4NCAR]

Originally posted by Avant�ix
quick question guys...

is the rolling diameter of a 235/35 tire mounted on a 9.5" rim the same as if mounted on a 8.5" rim?

i need to know ASAP...

also as far as the differential goes for Quattro is running the following combo okay:

front: 19x8.5" - 235/35
rear: 19x9.5" - 235/35 or 245/35

this is what i get from the Miata Tire Size Calculator:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/35-19 3.2in 12.7in 25.5in 80.0in 792 0.0%
245/35-19 3.4in 12.9in 25.8in 80.9in 783 1.1%

Wheel width should have no effect on the tires overall Dia. You can get the OD from the tire manufacturer though (each will vary slightly +/- 1mm perhaps with same size). If you're within some very small % (3%? IIRC), you're fine. Personally, I'd stick to the same all-around.

[Kat_Somm_Faen]

Anyway, you can always get the tires shaved :)

[Jet Jockey]

Originally posted by A4NCAR
Wheel width should have no effect on the tires overall Dia. You can get the OD from the tire manufacturer though (each will vary slightly +/- 1mm perhaps with same size). If you're within some very small % (3%? IIRC), you're fine. Personally, I'd stick to the same all-around.

Oh yes they do my friend. If you take a 235 and put it on a 8.5 inch wide rim and take the same tire and put it on a 9.5 inch rim it will change the rolling diameter because it is stretch out more across the wider wheel, thus it is lower.

I agree with Greg that the difference is negligible most likely around 1% and this apparently is well within the Quattro/TorSen working parameters. However, Greg's claim of 5% is a bit high according to my sources. I was told 3% which is still a lot.

Now the only reason one would want to get wider rims at the rear is for "looks". The staggered look is/may be only detrimental to the handling of the car. If the car sticks more in the rear because you have more rubber on the road, the car will understeer more. The question then is, why would you do that when Audi’s are already known to understeer?

[bbaudi382]

for the looks

[A4NCAR]

Originally posted by Jet Jockey
Oh yes they do my friend. If you take a 235 and put it on a 8.5 inch wide rim and take the same tire and put it on a 9.5 inch rim it will change the rolling diameter because it is stretch out more across the wider wheel, thus it is lower.

I was actually thinking that after I posted. Thanks.

[Feeesh]

i got a question. i'm planning to eventually go 18x8.5 kinesis k28s.... can vag-com recalibrate my speedometer? or the dealership? or whoever installs the wheels/tires?

[AvanTTix]

Originally posted by Jet Jockey
The question then is, why would you do that ... ?

b/c it looks cOOOOOOOOOOOOOOOOOOOOOl...

[3ohAvant]

Originally posted by Feeesh
i got a question. i'm planning to eventually go 18x8.5 kinesis k28s.... can vag-com recalibrate my speedometer? or the dealership? or whoever installs the wheels/tires?

why do you need to recalibrate?
just get the right size tire...

[Jet Jockey]

Originally posted by Feeesh
i got a question. i'm planning to eventually go 18x8.5 kinesis k28s.... can vag-com recalibrate my speedometer? or the dealership? or whoever installs the wheels/tires?

Why bother, your speedometer is already "OFF" by a small margin from Audi. It's done on purpose, something to do with German car certification. If you were to drive at a steady speed of 100 mph indicated on your speedo, you would most likely be doing 97 mph. Try it one day. Go on the highway, get a GPS, set you cruise control at 70 mph and see the difference. Another way to do it would be to time yourself over a mile run while driving at a steady speed, you would see the difference. Yet anothe way do see the difference is with a Vag-Com. There is a block that gives you a speed reading and I can assure it will not match your speedometer's indicated speed.

I would not worry about tire size.