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

[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

[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.

[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.

[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.

[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.

[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.

[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.

[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.

[Chris916]

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

[race2win]

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

Good point, however that was beacuse the Hoosier 275/15 wasn't available at the time, from what I remember he switched to the 15s when they became available.

No question, if I was building a DSP E30...or ANY sub-2500 lb street prepared car, I would be on the 15" A6s.

Also, getting heat into an A6 is a whole different animal that getting heat into RS3s. A6s work dead-cold & can easily OVER-heat...RS3s take some effort to get up to temp, & pretty much NEVER overheat.

[Nsquared97]

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

Race2win beat me to it. Your comparing 2 completely different tire compounds, obviously an R-comp designed for autoX is going to heat up faster than a street tire.

[CrazyB]

Good point, however that was beacuse the Hoosier 275/15 wasn't available at the time, from what I remember he switched to the 15s when they became available.

No question, if I was building a DSP E30...or ANY sub-2500 lb street prepared car, I would be on the 15" A6s.

Also, getting heat into an A6 is a whole different animal that getting heat into RS3s. A6s work dead-cold & can easily OVER-heat...RS3s take some effort to get up to temp, & pretty much NEVER overheat.

This is a great point, but RS3 do overheat. Trust me if you have a warm day and 2 drivers in the car the things do start to melt.

[BlackbirdM3]

Where I do most of my autoX, everyone has issues getting heat into the tires. It doesn't matter if you have a dune buggy or a Vette, they have issues with it.

For what its worth, I won my class yesterday and had the 2nd fastest BMW there, the guy who beat me was on R compounds with two drivers. I was on RS3s.

Will

[CrazyB]

A quick update, finally got the wheels fitted.
17x9 with et 20 running rs3 in 255/40/17 with 10mm spacers all around
Rears barely rub on the flares so have to grind the inner lip of the flares a little
fronts need a lot more work as the sheet metal under the bumper catches the tire barely on turn in. but looks like with a little TLC they should fit.




PS: rear fenders are a real pain to cut.

[Chris916]

Looks great! What wheels are those? And what is the weight of them? I just ordered a set of 255/40/17 rs3, and just about to order the xxr 527's to mount them on.

And yes the rear fenders are a real pain with the 2 layers to cut through.

Ask me how i know :P