Why no interest in 235/40R17?

[Greg_STL]

I ran some simple numbers on +10mm and +20mm on both 17" and 18" wheels and the 235/40R17 seems like the best fit for plus up size on the stock wheel/tire combo. I discounted the 235/35R18s because they have too small of a sidewall. Compare on the table below between the two more popular 225 sizes and the 235/40R17 seems better to me.

You would need to run a 17X8 inch wheel like the rota torques (17x8ET48) and there is still a good selection of tires in the 235/40R17 range. The big benefit to me would be a slightly smaller dia overall which means you are not losing hp to circumference and if you pick tire and rim well, you could lose weight as well.

http://www.goodmanstl.org/tire_compare.jpg

So, have people already measured and they don't fit the wheel well or what?? I can't have been the first person to run this calculation - what am I missing?

[SUBE555]

Tire dimensions are not so cut and dry. Nearly every tire is different in multiple ways. Some 225/45R-17 may be about as large as the largest 215/45R-17 or they could be on opposite ends of the spectrum. There are a lot of variables.

More than 225 width tires really won't get you much more if any traction. Good tires in even the 215 width will get you about as much traction as the car will require. Adding more will increase rolling resistance, friction, and help reduce mileage as well as sap away extra power. They may also be a very tight fit.

Just because you may fit a 17x8 wheel, that doesn't mean a 235 tire is necessary. Actually a 225 with a more square sidewall would be optimal for handling as the more square the sidewall, the better the tires will wear and the better footprint you will have.

Good rubber is good rubber. Size isn't everything. It's how you use it. ;)

[Xenonk]

A Falken Azenis 225/45/17 is built bigger than some 245 wide tires.. 235/40/17 is also not a common size.. a 245/40/17 or 255/40/17 is more common. All tires a built different as Sube555 states.

Another aspect that Sube555 forgot to mention is using the ideal rim width that fits the tire squarely.. so that the sidewall is more in-tuned with the rim. Oversizing the tire to the rim will make the sidewall weak.. undersizing the tire to the rim could stretch the sidewalls to roll over easily, and could possibly put a lot stress on the bead of the tire.

Keefe

[Greg_STL]

But its not width that I'm trying optimize, it overall diameter. I still say something close to stock dia or slightly less is better - assuming it fits the well.

From Tire Rack is you go into the specs for each tire:

Bridgestone S-03 235/40-17 Dia = 24.5
Eagle F1 GS3 235/40-17 Dia = 24.4
Michelin PS2 235/40-17 Dia = 24.4

All the tire specs also show that a rim width of 8" minimum for tire at that size.

[SUBE555]

Thanks Keefe, I didn't go into that enough.

My question is, do you already have a set of wheels that are 17x8" or are you looking around for another wheel and tire combo?

I know not too many people advocate 18" wheels, but just for comparison sake, I am comparing the Toyo Proxes 4's (which I use on my car) and stock 215/45R-17 size they are 24.8" dia, the 215/40R-18 size is also the exact same size. 225/45R-17 and 225/40R-18 of the same make and model of tire are 25.1" dia. The 215 models can take a 7-8" wheel, while the 225 models the same on 17" and one inch larger on 18". Not all tire mfgs are this uniform, but it's a good comparo. There are a lot of variables.

If you want to have a more square (better handling setup), choose a tire first, then pick the wheel you want to use with it, use the widest recommended wheel with that tire. Choose a specific size, and match the wheels. It may be a little harder, but the results will be worth it.

[Val]

Tire dimensions are not so cut and dry ... Some 225/45R-17 may be about as large as the largest 215/45R-17 or they could be on opposite ends of the spectrum. There are a lot of variables.

More than 225 width tires really won't get you much more if any traction. Good tires in even the 215 width will get you about as much traction as the car will require.

With all due respect, both paragraphs make little sense. Regarding the first one, in talking generalities one should be thinking "all else being equal" and not that some tires of the same size are larger than others. Otherwise, if everything is relative you couldn't compare anything.

The second paragraph flies in the face of the tire sizes used by the overwhelming majority of performance cars. A modified Legacy with 300HP or more would undoubtedly benefit from a 235/40-17 tire, provided it fits. That you should also use a good tire is just icing on the cake.

I agree with the original poster and I have said on another thread that Tirerack's 225 on 18" recommendation is wrong.

[SUBE555]

Well, unless you are making gargantuan amounts of power and have a much harsher ride, you shouldn't need 235 rubber. I'm talking probably levels approaching 300whp. With tires much better than RE92s, tires that have real traction, you should have a fairly hard time going beyond the limits of adhesion.

Go above those power numbers and 235/40R-17 may help, but below, you're not really doing very much help. That's if you can get those tires to fit without rubbing too. IIRC, most 235/40R-17 will not fit the wagons, may fit sedans, but rubbing is a very pressing issue without some modifications like rolling the fenders. 225 rubber should work for MOST, there are always those who really want more, whether it be for looks or actual function.

I think the reason that tire size is recommended by TR, most Subaru's (non-offroad models) have been using an aprox 25-25.1" dia and the 225/45R-17 and 225/40R-18 are about that size. They are also sizes that offer plenty of tire options. Not so with the tire that comes on the Spec B wheels. They are SEVERELY limited! Wouldn't be good suggesting that size.

As per my original comment you quote, what I was trying to say is you may have some 215 tires on the larger side that are nearly equal in dimension to smaller 225 versions, comparing in particular 215/45R-17 and 225/45R-17. Depends a lot on the mfg and varies by tire model. Look at the Bridgestone site for a good example IIRC.

So, while 235 'MAY' fit, it won't offer the same results in upsizing as a more powerful car powering 2 wheels. Most won't even come close to finding the limits of good rubber in 215 or 225 width rubber.

[Xenonk]

you do know that if you go with a shorter overall diameter, you do get back some of the lost torque ;) So why not just get 225/40/17? you might even lose 1 lbs in tire weight as well.. and if you are really in dying need to get the car closer to stock overall diameter sizing, I say inflate the tires a little bit more and drive the car harder to warm the tires up. :p

Keefe

[Greg_STL]

Xenonk-

What follows is a long explanation of why I didn't consider 225/40R17s.

Basically, when I ran the numbers I tried to find the closest aspect ratio number (35,40,45,etc) for a given width. In general, you get one too high and one too low for a given width.

Compared to baseline of 215/45R17:

225/40R17 -2.2% in dia
225/45R17 +1.4% in dia
235/40R17 -0.9% in dia
235/45R17 +2.9% in dia
225/35R18 -1.7% in dia
225/40R18 +1.9% in dia
235/35R18 -0.6% in dia
235/40R18 +3.2% in dia

I threw out anything over 2% off in case I needed to use the spare. The general thought is that on AWD cars that being off more that 2% will kill your differentials. (Random thought - I assumed the spare was very close to the dia of the 215/45R17 but I should check that.) I also threw out the 35 series because I fear for my rims. That left me three viable diameters in my study. Two are being reported as used by many, one by no one I had seen. So, I asked the question.... Honestly, I expected to hear back that they rubbed and I would throw them out of my study too. Instead, we are still here.

I have settled somewhat on the new Michelin PS2 as my prefered tire. I plan on putting snows on the stock rims. The PS2s are light, sticky and have gotten good reviews. The only down fall is price and for good tires I don't mind spending the money - just a personal thing. The PS2s are available in all three sizes that I'm looking into and are reported by Tire Rack to be the same diameters that I calculated.

I don't like the fact that the 18s are probably going to be heavier as well as having a larger rotational hit for increased dia of the tire size by almost 2%. I do like the fact that although the 235/40R17 will be heavier than the 225/45R17 it is liable to be offset because the tire dia is 2.3% (1.4%+0.9%) less on the 235/40 than the 225/45.

So, after all that, my real question seems to be: does anyone know if the 235/40R17 Michelin PS2 on an 17X8ET48 would fit on the GT sedan without rubbing?

*edited for spelling*

[apexjapan]

Just a quick, non-numbers related point to add to the discussion -

A really sticky 215 series summer tyre will wallop a sorta sticky high mileage oriented 235 series tyre. Sube-a-go-go-go (555 in japanese ;) ) has a point. I've participated in one tyre test with three diffferent manufacturers, one of which was in 225 series vs. 215's on the others, and in terms of traction at the track, there wasn't much into it. Add in the fact that at the amateur racing level, driver skill is a much, much higher factor than tyres - if you lose at the track, it is far more likely because of your skill than tyres or suspension setup. Witness the Ferrari 355 track and modena that I lapped at one 30 minute session. In my old B4 twin turbo with little in the way of modifications.

I wouldn't suggest that you cannot go with 235's, if and when I have to compete at a level where tenths of a second are going to matter, I will look at them for certain. But to be honest, I'd be stupid to not be using some sort of S tyre set that I reserve for track racing only - and enjoy the quiet ride and 4 plus mpg difference of riding on the 215 series RE050A's that handle more than adequetely for the street and winding roads, plus don't increase stress on the front steering and keep the fantastic turn in.

Cheers,

Paul Hansen
www.apexjapan.com

[brady_bunch]

Thanks for this thread and the knowledgeable comments... you may have just saved me from my own misconceptions about tire sizes, traction, and such. I was going down a similar path as Greg with 18" rim sizes, thinking 235s would provide better traction.

Hopefully somewhat still on topic... I was looking at some 18" rims on a BMW earlier in the week and noticed there was more of a "deep dish" space between the edge of the rim and the face of the rim. It looked cool but was there any point from a performance standpoint? Or is it just a matter of style?

[Xenonk]

Xenonk-

What follows is a long explanation of why I didn't consider 225/40R17s.

Basically, when I ran the numbers I tried to find the closest aspect ratio number (35,40,45,etc) for a given width. In general, you get one too high and one too low for a given width.

Compared to baseline of 215/45R17:

225/40R17 -2.2% in dia
225/45R17 +1.4% in dia
235/40R17 -0.9% in dia
235/45R17 +2.9% in dia
225/35R18 -1.7% in dia
225/40R18 +1.9% in dia
235/35R18 -0.6% in dia
235/40R18 +3.2% in dia

I threw out anything over 2% off in case I needed to use the spare. The general thought is that on AWD cars that being off more that 2% will kill your differentials. (Random thought - I assumed the spare was very close to the dia of the 215/45R17 but I should check that.) I also threw out the 35 series because I fear for my rims. That left me three viable diameters in my study. Two are being reported as used by many, one by no one I had seen. So, I asked the question.... Honestly, I expected to hear back that they rubbed and I would throw them out of my study too. Instead, we are still here.

I have settled somewhat on the new Michelin PS2 as my prefered tire. I plan on putting snows on the stock rims. The PS2s are light, sticky and have gotten good reviews. The only down fall is price and for good tires I don't mind spending the money - just a personal thing. The PS2s are available in all three sizes that I'm looking into and are reported by Tire Rack to be the same diameters that I calculated.

I don't like the fact that the 18s are probably going to be heavier as well as having a larger rotational hit for increased dia of the tire size by almost 2%. I do like the fact that although the 235/40R17 will be heavier than the 225/45R17 it is liable to be offset because the tire dia is 2.3% (1.4%+0.9%) less on the 235/40 than the 225/45.

So, after all that, my real question seems to be: does anyone know if the 235/40R17 Michelin PS2 on an 17X8ET48 would fit on the GT sedan without rubbing?

*edited for spelling*

I dont know if you ever seen my pics of me having 255/35/18 on 18"x8" with a +51mm offset.

You'll soon realize if you are going to make the best out of the car, I would lengthen the gearing by going with the larger sized tires. Finally getting 2nd gear to hit 61 mph is a good feeling and not needing to shift so fast. You can always go with a lighter rim and tire. Running around with just two different types of tires on the same tire "size" will throw the AWD out of whack.. Your differential will be fine even if have to use the spare at +/- 2%.. just be good about changing the differential fluid if you have to drive that far back on the donut. I dont understand why people get this fear about the absolute aspect ratio number.. if you get a tire wide enough, the aspect is nothing more than the portion of the width.. I run all sorts of tires on my GT.. from 215/45/17, 215/40/17, 225/45/17, 245/40/17, 225/40/18, and 255/35/18... I am at the point that I might even have some fun for a set of 285/30/18 of Hoosiers for autocrossing.

So for your answer: Yes, your choice of setup on 235/40/17 will fit on a 17x8 +48mm offset without any rubbing issues.


Keefe