Need advice on aftermarket wheels for c2s- currently running carrera classic oem tires- F 8.5 x 20 ET +51; R 11 x 20 ET +70.
Planning on 305's in rear with new setup.
C4s carrera classics use same F as c2s but rear is 11 x 20 ET +52.
Aftermarkets are F- 8.5 x 20 ET +44; R 11 x 20 ET +55 and are approved for 991.
Am i correct that front will be further in towards the hub by 7mm and rear will be away from hub 3mm using 305 tire (like c4s). If accurate, it seems a 7mm spacer would bring me back to stock wheel position (relative to wheel well) and I would need to place a larger spacer (say 12 or 15) to achieve flush effect to wheel well in front.
The rear would gain 3mm from stock wheel position so a 10mm spacer would likely be required to achieve a flush effect relative to wheel well.
Seems odd I would need larger spacer up front vs rear as I've never seen anyone note this type of setup.
Please help!
Thanks,
Rob

 

For any interested, I found a good wheel offset calculator (willtheyfit.com) to help with calculation.
Briefly, front aftermarket wheels are 7mm further out towards fender than oem Carrera classics; rear are 15mm further out toward fender than oem Carrera classics( I compared c2s rear oem offset to aftermarket offset going from 295 to 305's).
I found it interesting that these are the same dimensions commonly discussed for spacer widths. Clearly, the aftermarket wheels manufacturers are aware of this and incorporate in the design.

 

With aftermarket wheels at those offsets you will not require spacers. The smaller the offset number, the further out the wheels will come out of the wheel well.

 

Aftermarket offsets usualy are very aggressively adjusted, for stock wheel & tire widths.
If you widen your wheels and tires, you need to dial back the offsets accordingly.
In other words that rear offset may be too agressive for a 305, especially on a wider than stock wheel.
I dont think people with 305's would run a 15mm spacer, unless they like the look of tire sidewalls outside the fenders.

 

So if the aftermarket rear wheel is same dimensions as stock for both c2s&c4s (11x20) and a 305 is "stretched" on the wheel, wouldn't the offset of the aftermarket wheel just be the equivalent of a 15 mm spacer in the rear with stock rear wheels?
It seems from other posts, some have used 15mm spacers in rear without issue( maybe a little paint rash in rear). Would this be correct?

 

Assuming stock offsets and comparing to a 295, a 305 on;

• 11" wheel is equivalent to a 9mm spacer
• 11.5" wheel is equivalent to 11.5mm spacer

So yes a 15mm (but IMHO 12mm) spacer works on a 295 (with stock offset) but on a 305 you want to stick to a 5mm or 7mm offset change (depending on wheel width).

If getting aftermarket wheels, adjust your offsets accordingly to your desired tires section-width on the wheel-width they will be mounted on.
Just saying it all has to be accounted for as a system.

 

To answer your question.

The OEM per your original post is C2S
20x8.5 ET51
20x11 ET70 (ET52 C4S)

Your new wheels
20x8.5 ET44 (7mm out - towards the fender from factory)
20x11 ET55 (15mm out - towards the fender from factory) - But 3mm In towards the Hub from Factory C4S)

So yes you are technically in the rear 15mm more aggresive (as if you are running a 15mm spacer in the rear, but usually that is with a 295, with a 305 - you might see a hint of rubbing.)

Typically, what we end up doing is the following with a flush look without rubbing.
20x8.5 ET45 - 245/35/20
20x11 ET60 - 305/30/20 (or 295)

Here is a perfect example of that setup. This is a 991 C2S on stock Suspension (PASM Equipped) and running the 305's in the back on an HRE P40SC. This car is 5mm Less Aggressive in the rear than what you are looking to do, and 1mm less aggressive in the front.

Feel free to give us a call if you have any questions about widths and offsets on the 991's, i would be more than happy to help out!

 

If going with 305's on rear then suggest 1/2" wider rear wheel than came with the car (other than with Sport Techno wheels).

 

Don't stock c4s Carreras classics come as 11x20 with 305's ? Are you suggesting that setup due to tire stretch or is there another reason?

 

For the simpleton such as myself, I think you are saying if the aftermarket offset is ET +55 in rear on 11x 20 wheels and I run 305's it would be equivalent of a 9 mm spacer on stock oem Carrera classics on c2s running 295's. Do I have it correct?
If so, it sounds as if you wouldn't recommend that combo due to possible tire rub.

 

Thanks for requesting the clarification.
I was referring to a 9mm offset change, just from the 305, on a stock [70mm] wheel.
The tire section width changes from 11.8" to 12.5" (18mm difference).
So you also get 9mm of tire moving inboard also.

If you put a 305 on on a 11" +55mm wheel;
that is the equivalent of a 24mm spacer vs srock [295/11"/70mm].
Thats assuming PZero tires.
It's slightly less (by 2.5mm) with a PSS, since it has a 0.2"/5mm smaller section width.

So as you can see, typical aftermarket offsets don't let you run wider tires and wheels.

The 305/11"/60mm, that Tag mentioned above, would be the equivalent of a 19mm spacer vs stock.
(Again, assuming PZero as a PSS would be 2.5mm less)

 

So, after much remedial work on my behalf, the aftermarket offset +55 on 11x20 is fixed at 15 mm out toward the fender. Adding a michelin 305 pss with section width of 12.3" adds approx 0.7" or 17.5mm of which half is outer portion of wheel and half inner portion of wheel.
I've spoken to the aftermarket company who notes they have used this exact setup on a promotional c2 (using pirrreli p zero nero with section width 12.3) AND lowered the car with no rub.
Based on calculations that's equivalent of 21.5 mm spacer on oem c2s with pasm. That sounds extreme. I'd be curious to know what's the largest spacer used without rubbing in a non lowered c2s.