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Here's my situation: I'm looking for tires for 17x7.5 wheels for use on my 2004 WRX.
Tire Rack recommends 225/45-17 but in the May 2007 issue of GRM they used 235/40-17 on their Saabaru.
Which would be the best size for dedicated summer street/HPDE WRX tire?

extra credit question: why?

I think I know, but I'm the kid in the back of the class who's just going to listen and see if he's right...

Let me take an educated guess.

235/40 is wider (for better grip), stiffer (less sidewall flex for better handling), and little lower profile than the 225/45 so that you will accelerate slightly faster(although the speedo will be off by 2.3% meaning it will show you going faster than you really are but not by much...also better for getting less speeding tickets, no?...also have a slightly slower top speed). So basically better if you will be hitting the track.

As far as pricing/availability, not sure but I would think the 225/45 would be more common place, more tire choices and possibly cheaper...

Are getting some nice, lighter wheels for these tires?

thanks for the info, Will! Tires are a weak spot in my car knowledge.

the 225/45-17 size is between $30 and $50 cheaper per tire for the same model on Tire Rack's site. I guess since these will be dedicated summer/performance tires I might spring for the extra width and speeding ticket resistance.

as far as wheels, I'm looking at a set of

There are a number of trains of thought here. One says to get the widest tire under you you can get and thus have as big a contact patch on the pavement as possible. The second is that rolling weight should not be increase just for the sake of a slightly larger contact patch. The third is that you might actually want to reduce the diameter of your tires to lower your final drive ratio slightly.

Let's look at some figures:
Bridgestone Potenza RE-01 R 235/40 ZR17
section width: 9.5"
weight: 25 lbs
diameter: 24.4"

Bridgestone Potenza RE-01 R 225/45 ZR17
section width: 8.9" (.5" smaller contact patch)
weight: 26 lbs (1 lb heavier)
diameter: 25" (.6" larger)

It would appear that the 235/40s would be a no brainer, at least as far as the RE-01 Rs go, except for 1 issue. The specs for the 235 claim a rim width range of 8-9.5" where the 225 say 7-8.5", so if you don't have a set of 8" wheels you may not be able to mount the 235s. Also, you can expect that 8" wheels will generally weigh at least 1lb more than their 7.5" conterpart so you just gained back the lb you lost.

One more thing needs to be said. The numbers don't always add up. Take for example my current set of tires:

Kuhmo ECSTA SPT
215/40 ZR17
section width: 8.6" (only .1" smaller than the 225/45 17 SPT)
weight: 21 lbs (4 lb lighter than the 225/45 17 SPT)
diameter: 23.8" (1.2" smaller than the 225/45 17 SPT)

I got nearly the same size contact patch, while dropping 4 lbs per tire and reduced my diameter by 1.2" which brings my final drive ratio clser to 4.1 which is a plus.

Did I muddy the waters enough for you?

the doctor is IN

drwrx wrote: It would appear that the 235/40s would be a no brainer, at least as far as the RE-01 Rs go, except for 1 issue. The specs for the 235 claim a rim width range of 8-9.5" where the 225 say 7-8.5", so if you don't have a set of 8" wheels you may not be able to mount the 235s. Also, you can expect that 8" wheels will generally weigh at least 1lb more than their 7.5" conterpart so you just gained back the lb you lost.

Sorry Darrin, but I have to call you on this one. 7.5" wide is the minimum for 235's which I am running no problem on my T2's that are 7.5" wide.

Running 235's on 8" wides will net you a slightly tighter/lower sidewall (but not very much) vs. running 235's on a 7.5" wide.

I would suggest the 235/40/17's on this one on 7.5" wide vs. 8" to reduce the overall wheel weight.

sirwilliam wrote:Let me take an educated guess.

Phew... this is exactly my explanation. Now I don't feel like an idiot for the day.

sirwilliam wrote:Sorry Darrin, but I have to call you on this one. 7.5" wide is the minimum for 235's which I am running no problem on my T2's that are 7.5" wide.

Running 235's on 8" wides will net you a slightly tighter/lower sidewall (but not very much) vs. running 235's on a 7.5" wide.

I would suggest the 235/40/17's on this one on 7.5" wide vs. 8" to reduce the overall wheel weight.

You certainly may be able to run 235/40s on 7.5" wheels. I only said that, because the RE-01 Rs specs (and all the others of that size as well) specified a rim width of 8-9.5". I certainly did not mean to say that it was an absolute. However, it may be that, with a section width of 9.5" and a very stiff sidewall, you may not be able get them to fit on a 7.5" wheel. Also, to be clear, you are running P-Zero Nero 235/55/17 which have a section width of 9.2" and have a significantly taller (read less stiff) sidewall than a 235/40/17 and the P-Zero Nero 235/55/17 specs indicate you can run them on 7-8.5" wheels.

225/45-17 it is! thanks to all that have contributed.

I will say that the GRM Saabaru was running 17x7.5 rims IIRC. I'll double check tonight and correct this if I'm wrong.

drwrx wrote:

sirwilliam wrote:Sorry Darrin, but I have to call you on this one. 7.5" wide is the minimum for 235's which I am running no problem on my T2's that are 7.5" wide.

Running 235's on 8" wides will net you a slightly tighter/lower sidewall (but not very much) vs. running 235's on a 7.5" wide.

I would suggest the 235/40/17's on this one on 7.5" wide vs. 8" to reduce the overall wheel weight.

You certainly may be able to run 235/40s on 7.5" wheels. I only said that, because the RE-01 Rs specs (and all the others of that size as well) specified a rim width of 8-9.5". I certainly did not mean to say that it was an absolute. However, it may be that, with a section width of 9.5" and a very stiff sidewall, you may not be able get them to fit on a 7.5" wheel. Also, to be clear, you are running P-Zero Nero 235/55/17 which have a section width of 9.2" and have a significantly taller (read less stiff) sidewall than a 235/40/17 and the P-Zero Nero 235/55/17 specs indicate you can run them on 7-8.5" wheels.

Sorry, I didn't know that we had narrowed it down to only one type/brand of tire. Then, you are correct.