225/45/15 vs 255/40/17 RS for autox

[Chris916]

Obviously the solution here is to just step it up to 275/35-15 A6's ;)

[Bimmerman325i]

Alex Schipkov's DSP E30 car ran 18x11s with 285/30/18 A6s, so getting heat into a large tire is a non-issue.

[Nsquared97]

I did. E36s have done extensive testing, majority of fast cars I know of run 255/40/17 on 17x9 or larger. That goes for M and non-Ms, so it's not a brake clearance issue.

Would a 275/15 be faster? Likely so. However, OP wanted to know for the RS3 specifically, which does not come in that size.

FWIW, I referenced the E36 cars based on weight...they are larger, heavier, have different suspension geometry, have different weight distribution, etc.

IMHO, there is a "sweet spot" in regards to tire-to-weight ratio...exceeding it has diminishing returns due all the trade-off factors I mentioned. This is why STS/CSP Miatas & ST Civics run smaller tires than what is allowed in those classes. Yes, I know the 195 Toyo used in ST/STS is different, but even if it had the same compound as the 225s they would still be on the 195s for all the same reasons.

So again, even though I do agree the 255/17s "should" be faster, it's not a slam-dunk as some are saying for an E30. Just because they are best on an E36 does NOT necessarily mean it's best for an E30.

^This is what I was referring to. They may be the best for a E36, but that doesn't mean they are the best for an E30. Weight, and the ability to get heat into a tire (espicially on a short autoX course) make a big difference. There's a reason why light formula cars don't run huge steamrollers: They can't generate enough heat to make super wide tires effective. The couple hundred pound weight difference between an E30/E36 may be enough to effect that.

[race2win]

I did. E36s have done extensive testing, majority of fast cars I know of run 255/40/17 on 17x9 or larger. That goes for M and non-Ms, so it's not a brake clearance issue.

FWIW, I referenced the E36 cars based on weight...they are larger, heavier, have different suspension geometry, have different weight distribution, etc.

IMHO, there is a "sweet spot" in regards to tire-to-weight ratio...exceeding it has diminishing returns due all the trade-off factors I mentioned. This is why STS/CSP Miatas & ST Civics run smaller tires than what is allowed in those classes. Yes, I know the 195 Toyo used in ST/STS is different, but even if it had the same compound as the 225s they would still be on the 195s for all the same reasons.

So again, even though I do agree the 255/17s "should" be faster, it's not a slam-dunk as some are saying for an E30. Just because they are best on an E36 does NOT necessarily mean it's best for an E30.

[Bimmerman325i]

Read the post above yours.

I did. E36s have done extensive testing, majority of fast cars I know of run 255/40/17 on 17x9 or larger. That goes for M and non-Ms, so it's not a brake clearance issue.

Would a 275/15 be faster? Likely so. However, OP wanted to know for the RS3 specifically, which does not come in that size.

[noid]

http://www.rtsauto.com/calculating-rim-and-tire-sizes/

You are talking about jumping from a 22.97" wheel diameter set up to a 25.03" wheel diameter.

Your best bet is to run 255/35/17 to be at stock wheel diameter. Or 255/30/17 to be at your current wheel diameter. To bad neither of those sizes are virtually non existent.

[Nsquared97]

For E36s, 255/40/17 on a 17x9 is the fastest street tire setup.

I can't imagine E30s being any different in that regard.

Read the post above yours.

[CrazyB]

Also I am planning of swapping an s50 or s52 into the car in a year or so, at that point this new wheels will also be a much better match. Now Sm land should be an interesting place.

[CrazyB]

All things being equal, more tire on the ground will create more ultimate grip. The trade offs in this situation are more un-sprung weight, increase in track width, taller gearing & being able to make them fit. According to your statements above, the latter 2 trade-offs won't be an issue, but the first 2 will offset any gain at least to some extent. Plus, as was mentioned before, on colder days it will be more difficult to get heat in RS3s that large in a lighter car. Yes, the E36s run that size very effectively, but those cars are at minimum 2-300 lbs heavier. Keep in mind, CSP Miatas can run ANY wheel-tire combo...they could run 315/18s if they wanted...but they instead run 275/15s because they have proven to be the fastest package for that particular car. So bigger is not neccessarily always faster

Summing up, over-all the 17s should be faster 95% of the time, with the 15s potentially slightly better in very tight & transition heavy courses. The only way to find out for sure though is to test, back to back, on the same day & in the same conditions. I'm sorry, but to all the guys saying the 17s will for sure be faster, you will never know for sure without doing back to back testing. More to the point, even if they ARE faster, you will never know WHY or HOW they are faster. My $.02 :)

I guess I will need to do some testing this season, I currently have 15x8 with 225 rs3 so once I get the new wheels an tires i will see if i will have the time to try and run them both on the same course or its time to wait for a test and tune day.

As far as people talking about stuff like fuel efficiency i am not too worried the only driving this car does on the street is to the AutoX and Back.
Well and a nice spirited canyon drive when the weather is good

[Bimmerman325i]

For E36s, 255/40/17 on a 17x9 is the fastest street tire setup.

I can't imagine E30s being any different in that regard.

[race2win]

I currently have a 3.73 in the car, but I have a 4.10 seating in the garage also so that should change the gearing if that becomes an issue. As far as clearance I am fine with cutting the fenders and removing the liner, the car is not a daily driver.

I was more going to see if anyone ran this size and how the car feels on the larger tire.

All things being equal, more tire on the ground will create more ultimate grip. The trade offs in this situation are more un-sprung weight, increase in track width, taller gearing & being able to make them fit. According to your statements above, the latter 2 trade-offs won't be an issue, but the first 2 will offset any gain at least to some extent. Plus, as was mentioned before, on colder days it will be more difficult to get heat in RS3s that large in a lighter car. Yes, the E36s run that size very effectively, but those cars are at minimum 2-300 lbs heavier. Keep in mind, CSP Miatas can run ANY wheel-tire combo...they could run 315/18s if they wanted...but they instead run 275/15s because they have proven to be the fastest package for that particular car. So bigger is not neccessarily always faster

Summing up, over-all the 17s should be faster 95% of the time, with the 15s potentially slightly better in very tight & transition heavy courses. The only way to find out for sure though is to test, back to back, on the same day & in the same conditions. I'm sorry, but to all the guys saying the 17s will for sure be faster, you will never know for sure without doing back to back testing. More to the point, even if they ARE faster, you will never know WHY or HOW they are faster. My $.02 :)

[Chris916]

Bigger is always better ;) With my shitty m50 with a dragging cylinder, i was still running out of grip with 245 ra1's, 3.5 degrees of camber in the front and 2 in the rear. 255's on 17's would be magic. Most e36 guys run this setup and seems ideal as well.

[BlackbirdM3]

Another factor you should think about is getting heat into the tires. The bigger the tire, the longer it takes to get heat into it. I see you are here in the Bay Area, do you ever run at Marina? The surface is fantastic, getting heat into the tires is a different story.

Unless you have some monster motor, I highly doubt you are going to run out of grip with a 225 on your car. I'm running RS3s and I don't have an issue with too little grip, especially in the rear. My car was pushing, but I also wasn't using the inside of the front tires at all. Now that I've added lots of camber and caster to the front of the car, I don't think it will be an issue. As it was at the first event I ran the car at, it was the fastest BMW at the race, race tires or not. It should be a class winner this sunday. (Maybe not, its hard to say. I think I might have too much caster at this point the steering wheel doesn't self-center well with 10.5 degrees caster. I'm hoping its just a matter of tight new parts and the issue will go away, but likely not.)

Will

[team9]

I am currently running a 225 RS3 on a 15lb 15x8 wheels
For this season i was thinking of moving up to Rota Grids in 17x9 and run the 255/40/17 RS3
Anyone have experience with running the 255 tire and have any feedback.
One of the reasons i am making the jump is for next season i can run 275/49.17 r comps on this wheels also.

I know the fenders might not clear them but i am willing to add rivet on flares or custom flares to clear the rubber.

Just curious if anyone made the jump form 225 15 to a wider 17 inch tire and how the car felt.

DO IT!!! You won't regret it. I ran half the 2011 autox season on a 16x7 with 225 Kumho XS's and half the season on 17x8.25 front, 17x9.75 rear XXR's with 255 Kumho XS's. The difference was unbelievable! Definitely would recommend getting the rivet on fender flares from Ary at DTMfiberwerkz. Any deficit in unsprung weight is completely negated in my opinion. The first half of the season I was consistently ~2 sec slower than the other competition (modified evo's) in my local SM class. After I put on the 255's, I moved to within ~.5 secs of the leaders. Went from 4th and 5th place finishes to 2nd and 3rd. Proof speaks for itself. But this is just my opinion. I can't wait to try out the RS3's. From the articles Grassroots Motorsports has done on the latest autox tires, the RS3's far exceeded the performance of the XS's.

To give you an idea of the car setup:
Engine - cone filter, 19lbs injectors, IE shorty headers, 2.25 dual exhaust, no cat, vibrant muffler at rear
Suspension - Bilstein sports, GC coilover kit which includes GC camber plates, adjustable sleeves with Eibach race springs, 450/600 front/rear. Also have UUC Swaybarbarian sway kit 22/19mm front/rear.
Interior - removed rear seat and carpet, corbeau forza seats, corbeau harnesses, caged interior
Exterior - DTM fiberwerkz fender flares

Are you also interested in tracking the car? If so, I would still use the 255's on the track. IMO, any straight away speed is negated with the corner speed and increased corner exit speed. I say that with the exception of having a really slow corner into a really long straight. If that's the case, then you may see an advantage with a smaller tire and better gearing. But even if that's the case, you could make it all back on the rest of the track. I only have one track comparison between the 225's and 255's mentioned above. In March 2011, I ran at Talladega Gran Prix Raceway with a fastest time of 1:13.2. In November 2011, I ran again with the 255's and ran a fastest time of 1:11.2. Only differences between the two weekends was tires, and no passenger seat.

Take my experiences for what they're worth. In the end, I would do what I want to do. You'll never know if something is better or worse until YOU try it.

[Jaxx_]

Running a 16x7.5 with 225/50 Advan AO48s. I would rather use a smaller wheel and slightly smaller tires and run a good tire than run a big, wide, shitty tire.

I would say steering response is another factor, but if you're doing autox you probably already have a good alignment setup, yes?