More Wheel/Tire Sizing Madness... Need Help

[Jason_Els]

I am racking my brain over this. I want to upgrade the wheel/tire combo I currently have on my FXT. I checked the tire calculator at Miata.net and it seems to indicate that 235/50/17 is a better match than 225/50/17. The specs I indicate seem to show that:

Stock:

Sidewall: 5.1 in
Radius: 13.1 in
Diameter: 26.2 in
Circ: 82.2 in
Revs/mi: 771.0

225/50-17

Sidewall: 4.4 in
Radius: 12.9in
Diamter: 25.9 in
Circ: 81.2 in
Revs/mi: 780.0
= +1.1% speedo error

Now compare that to:

235/50-17

Sidewall: 4.6 in
Radius: 13.1 in
Diameter: 26.3 in
Circ: 82.5 in
Revs/mi: 768.0
= -0.4% speedo error

To me, it seems the 235s make more sense and stay more proportional than the 225s.

The really interesting setup though is with 18" wheels:

235/45-18

Sidewall: 4.2 in
Radius: 13.2 in
Diameter: 26.3 in
Circ: 82.7 in
Revs/mi: 766.0
= -0.6% speedo error

What stumps me is the offset allowance for each of these and if there would be rub. I'd like to upgrade later to the FSTI suspension which only drops an inch. Any thoughts on this? Am I just completely wrong? It doesn't seem that any of these combinations are so substantially out-of-spec that they can't be used with safety.

As for the offsets, I have no clue. Is there a rule for offset calculation when changing sizes?

Thanks!

(my apologies for the cross-post with subaruforester.com but I need an answer very soon as I need to get my studs off ASAP).

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[Peaty]

Yes it's a better match but people that have tried said the width of the tire makes it rub. Fender or strut I forget which.

[GuyLR]

I think 235 is more tread width than you need for a Forester. IMO a 225/45-18 would be the perfect size. Don't obsess about the 1% difference in circumference, you won't feel it. That will show up as 75 mph when you are really doing 74.3 mph. The 225/45-18 will give you the same sidewall height as an STI but with the near correct diameter for a Forester and no rubbing. And to top it off, the 225 will be a bit lighter than the 235.

Try this tire calculator for more info on ratios and the effects of different offsets.
http://gs.tolan-hoechst.com/tirecalc.htm

[Jason_Els]

Thanks Guy!

My worry is ordering a tire/wheel set only to have them arrive and not fit. If I'm right, I want to keep as close to spec while reducing sidewall but without rubbing.

[ferretboy]

Jason,
if your stock tire size is 215/60/16 then you can safely use a 225/45/17 with absolutely no rubbing problems as long as you use rims with a 48 offset. That is what I'm using on my 2001 and I don't think that there are that many differences between the two when it comes to tire sizing and clearance which is mainly an issue on the rears either with rubbing on the strut body or fender lip. I have even run 205/55/16 azenis mounted on 52offset sti version VI wheels with enough clearance (1/4" from inside of tire to the strut body. But hey what do I know?
~ferretboy

[RoundtheBend]

Teh Ferret,

You've got PM.

-Erik

[brett s]

Personally, I'm thinking about 225/55/17, or going with 225/60/16's on the stock wheels - the speedometer already reads slow (about 3-4%, verified by radar & gps), and even the factory size looks rather small in the wheelwells to me at the stock ride height. They'll be less than 1/2" taller, and the speedometer will still read slightly high.

Might be different if I was planning on lowering it, but that ain't gonna happen - I'll take the tradeoff of better ride quality over handling given how I spend most of my time driving. Driving 30,000 miles a year, mostly on concrete freeways... I really appreciate the XT after driving my wife's Infiniti G35 coupe, which handles great on the back roads but beats you to death on the highway.

[omahasubaru]

Originally posted by ferretboy
Jason,
if your stock tire size is 215/60/16 then you can safely use a 225/45/17 with absolutely no rubbing problems as long as you use rims with a 48 offset. That is what I'm using on my 2001 and I don't think that there are that many differences between the two when it comes to tire sizing and clearance which is mainly an issue on the rears either with rubbing on the strut body or fender lip. I have even run 205/55/16 azenis mounted on 52offset sti version VI wheels with enough clearance (1/4" from inside of tire to the strut body. But hey what do I know?
~ferretboy

You have no rubbing issues as your diameter is much smaller than stock with Impreza tire sizing on your wheels. It's when people try to maintain their speedo readings and want to go wider that they have rubbing issues.

Especialy if they drop the car trying to use an Impreza strut/spring combo as the rear spring seat gets in the way.

I would be leary of going over a 225 tire if it were me.

That said... depending on the suspension choice this time with my friend FXT (had a nicely lowered '98 Forester) we may go with a wider tire (I'm tryin to talk him into Tein's).

Thanks - Jon

[Bobshere]

I run 225/50R17 on my FXT. VERY happy with the mod and I can still use FXT "offroad". I think to add more is to favor looks over performance (for the utility of THIS car) and your wallet (remember those 18" tires are going to cost more EVERY time you buy a new set).

Cheers,

Bob

[sleeperXTc]

I'm soon to be lining up a second set of wheels (winter) and tires, and was thinking it might be a time when I could rationalize (to my better half) moving into 17 or 18 inchers...following this and other similar threads has been helpful!
Seems like there are two camps, those wanting to change for appearance preferences, and those wanting to change for performance preferences.
I've not had mag wheels before and am easily pleased, the stock XT wheels are fine. While I've been generally pleased with the Geolanders as a compromise (as with most compromises, the extremes get averaged out and there are few features that are more than adequate).
My own preference is towards performance (winter and summer, Alaska winters and Arizona summers), and I'll probably be looking for some winter 17's (not likely to be mag type, with winter rigors to face), and not that wide (I prefer tall/narrow tire for both control and traction reasons).
For summer set, I'll be looking to stay with the 16s, but will not get another set of Geolanders...will be "TireRacking" or "MazdaMiata-ing" for a performance tire for primarily pavement duty.
I've long been attached to Michelins and Pirellis for past sports car applications, but haven't kept up with "handling tires" much lately.
Any recommendations? I have made the Geolander squeal (even with 33/35 tire pressures). I've no suspension mods...

[speedypete]

I have a set of 225/55/16 bridgestones re750's on my stock wheels and they are the best tire i've ever had. It made a HUGE difference in handleing over the stock geolanders. Turn in is alot crisper and much improved steering feel. Ultimate grip is AWESOME. Tire rack has rated them in the top 3 for summer performance tires.

[conradfxt]

speedypete,

Thanks for the post about the 225/55/16 bridgestones re750 on your stock wheels. Will check local shops first but did look at the great reviews at the TireRack. What tire pressures do you have on the Bridgestones?

conrad

[lucien2]

Originally posted by sleeperXTc
I'll probably be looking for some winter 17's (not likely to be mag type, with winter rigors to face), and not that wide (I prefer tall/narrow tire for both control and traction reasons).
For summer set, I'll be looking to stay with the 16s, but will not get another set of Geolanders...

You've got it backwards, mate. Use the stock wheels for winter. Higher sidewall on the tire and skinnier patch are better for the snow. Use 17" rims for summer- lower sidewall height means less carcass deformation and crisper handling.

Oh, and I don't think anyone makes a mag(nesium) wheel for the street these days

[Bobshere]

There could be a point to using the OEM 16" wheels for performance and the newly bought 17"'s for "All Weather". As Lucien is correct about the sidewall issue, the 17"'s on the All Weather would boost their compromised performance. While a 16" Performance tire would not perform as well as a 17" one, there could be a tire wanted in 16" that is not available in 17". Cost of going one way or the other could be a factor as well.



There are a lot of permutations and factors to play with in this equation....

Bob

[bluesubie]

There's also a new choice in summer UHP tires in the 225/50/17 size.
Avon Tech M500 thread .

If you are staying with a 16" summer tire, there are even more choices.

-Dennis

[speedypete]

Originally posted by conradfxt
speedypete,

Thanks for the post about the 225/55/16 bridgestones re750 on your stock wheels. Will check local shops first but did look at the great reviews at the TireRack. What tire pressures do you have on the Bridgestones?

conrad

I just changed the pressures a couple days ago from 29 psi to 34 psi. No particular reason for the change except that I wanted to see how much the ride suffered with the higher pressure....very little difference in ride quality.

I only have 2100 miles on the car (about 1200 on the bridgestones) so I really haven't found the "right " pressure for my taste just yet.

[yvesberib]

I think I'm with brett s, and if I were to buy new wheels I'd go with 225/55 on 17x7. This will provide 0.4" more width and 2.4% higher speed at the same rpm. It will also lower the ratio of tire width to wheel width from about 1.3 to 1.26, which is supposed to help a bit.

But staying with the lighter weight (and free!) 16" wheel may be the way to go for me. Like brett s, I think I'd lean towards 225x60, but might consider 225x55.

From what I've gathered, though, a good high performance tire, even of the same specs as the Yoko OEM, would make a big difference. I like the reports on the Conti Extreme Contact.

[sleeperXTc]

Thanks for the input! Lots of things to consider. One of the comments I've heard is that you can lose 'compliance' with uneven road surfaces with the lower profile, stiffer tires (which, on a flat race track surface would be ideal, but I don't get near enough time on the race track...:>}