ThetaTau87

I recently added a supercharger and would like to get wider tires in the rear for more traction and increase the fronts as well to keep the balance. In the front I currently have 225/45/17 on 17x7 et55 wheels. At the rear I have 245/40/17 on 17x9 et55. These are very close to the stock 16" tire sizes.

I'd like to go up to a 245 in the front and 275 in the rear. I have a pair of 17x8 et70 wheels for the front and will be keeping the 17x9 et55 in the rear. Will 245/45/17 be too tall in the front with 17x8 et70? Same question at the rear with 275/40/17 on 17x9 et55? They are roughly .75" taller than the stock tire diameter.

The alternative sizes to keep closer to the stock diameter would be 245/40/17 front and 255/40/17 rear. That would decrease understeer, but might not give enough traction at the rear. I can spin the current 245/45/17 rear Pilot Super Sports pretty easily. I'd be replacing them with Conti ExtremeContact Sport.

FredR

The 245/45 is a better choice diameter wise as it goes nicely with the 275/40 rear selection.

Those tyres will work with the wheel sizes proposed and the rear section will probably give you a more accurate speedo reading as well as suiting the final ratio a bit better given the GT is often considered a bit undergeared with a stock motor yet alone force fed.

soontobered84

On my 89, I get a rub with 245/45 in the front, but I have used 275/40s in the back, and they were great.

ThetaTau87

A more accurate speedo reading with 275/40/17? It will read nearly 4% slower than the stock tire diameter. Are you saying that the stock speedo calibration is too fast and the taller tire will correct it and make it read closer to the actual speed?

When I've checked my speed with a GPS app the speedo reads nearly identical to actual speed with my current 245/40/17 rear tires. That tells me a taller 275/40/17 tire will make the speedo read slower than actual speed.

ThetaTau87

What size wheel and offset in the front?

FredR

I think it may be appropriate to make some points of clarification here viz:

1. When it comes to wheel fitment one can only generalise based on experience knowing that not all models are dimensionally the same to within minute quantities. I found slight differences side to side on my late 90 S4- have not measured my current 928 GTS chassis.
2. When folks like John say they had a rub with a specific tyre fitted that obviously reflects their experience with a specific tyre and specific wheel combo on their specific example. Such experience is a good marker. Most non stock wheels that folks use up front have an incorrect offset and try fitting a 245 section on such wheels will invariably result in a rub on the outer fender so good to ask the question "what actually rubbed and what was the offset".
3. Most probems occur because folks cannot obtain the profile they really need. In your case Conti do a 245 section with a rolling radius of 24.7 inches so I am not sure why you would want to go to a bigger profile if there is no real need. There has been plenty of discussion about how the ABS system can be impacted by different rolling radii but my experience to date with differences front to rear is that the system is tolerant to some extent. I currently run with 25.0 inches up front and 25.7inches diameter at the rear and have no issues so in your case I would go with a 245x40x17 section up front given an 8 inch rim can take this.
4. Regarding rolling radius on the rear wheel. Most speedos tend to be a bit optimistic, from a legalistic perspective they need to be. This was discussed in an interesting thread some time ago and one of friends in Scandanavia comments on how a slightly bigger diameter gives a reading that is more accurate. At the time I think we were talking GTS's that have a rear rolling radius of 25.0 inches compared to rubber with about 25.7 inches as I seem to remember. One has to remember that the rolling radius changes slightly with speed [or so I understand] so at typical legal speed limit of say 70 mph there may well be changes when the car is maxxed out. Either way, if you run with a non stock rolling radius good to calibrate what you have fitted but do so with what you know to be accurate- not all GPS devices will give you the same number yet alone an exactly correct number.

I spent a lot of time measuring the front geometry before fitting wider wheels to my 928. I concluded that if one was to do this then the stock ET65 was a minimum to run with. I also concluded that the anti roll bar [in my case a Devek bar] was likely to be the first rub point [it was] and to make a 265x35x18 front section work [25.4 inches diameter] I would need to fit an additional steering rack stop either side as proved to be the case.

Many folks have actually fitted bigger wheels and ended up with a slight rub but such inevitably take place a full lock and not really an issue when driving hard, usually being noticed in a car park when manoeuvering. Such would drive me crackers but that is me.

Trust the above helpful. At the end of the day you just need to make a decision based on what consensus opinions suggest but once one goes to non stock alternatives one has to take responsibility for one's decisions as I do.


Rgds

Fred

SwayBar

If you need grip, get the most rubber you can under it. On my '87:

Front - 9.5 x 18, 68mm offset - 265/35
Rear - 10.5 x 18, 59mm offset - 295/30

No lip-rolling required.

I'm thinking a 315 can fit up under there with that offset.

FredR

The OP is not going to get a 265 section on his 8 inch front rims

I concluded some time ago that a 265 front is probably too much rubber for the 295 to keep up with even with the Devek bar set at full hard so I can see where you are coming from if looking for more rear rubber - fine if you have 11 inch rear rims. Whether such will actually fit inside the S4 arch I am not sure but it will certainly be a tight fit. Your offsets are ideal- same as mine.

SwayBar

Fred, you are right; I've edited my post and added the rim widths.

FredR

One point often overlooked when trying to find/fit front wheels with a large offset is whether the spokes clear the calipers. Making such wheels with cast alloy is rather difficult thus why they are rare to non existent and when available tend to be rather heavy to get the necessary strength to deal with the overhanging moment such offset generates. Thus 3 piece rims with forged centres.

The other point is that when mounting such wheels you have to be careful not to trap your fingers between the spokes and the calipers- got the scars to prove that!

ThetaTau87

Thanks for the input. I'm considering the 245/45/17 to avoid the 1/2" rake that would be caused by taller tires in the rear. 245/40/17 are 24.7", 245/45/17 and 275/40/17 are both 25.7". If the 245/45/17 fits up front I would avoid any rake and have addition sidewall compliance to cushion impacts to prevent wheel bends.

James Bailey

The composition of the tires has more to do with traction than does the "width" plus note that the 245 or 275 is NOT tread width but the bulge widest part of the sidewall when mounted on a rim. And each manufacturer can choose the rim width for their measurement. A few will state actual tread width but that is mostly only on race tires.
Which makes comparing one 245 to another 245 very difficult.....

Randy V

245 front makes a noticeable improvement in turn in.

Currently have 275 rears, will go up to 285 or 305 on the next set.

VT928

The 928 is front axle is under tired, wider tires on the front will greatly improve turn in and braking. My car currently runs 265/40 17 on the front, with 17*9.5 60mm offset. The rears are 285/40, on the same wheel as the fronts. Changing tire and wheel sizes will impact more than just the contact patch, wider on the front tires tend to reduce understeer. Wider tire soffer a larger contact patch, bu can induce wheel hop.

AO

If you want grip, the other thing to consider is the tire compound. Getting a wider tire with a treadwear rating of 500 will likely have less grip than a narrower tire with a treadwear of 100, but you know that.