FAQ: Offsets, Sizes for Rims and Tires on the GT! Xenonk Version!

I finally compiled the most completed info I can come up with for wheel and tire fitment questions that everyone has. I mapped this entire post out so that you can pick a rim size you want and the rest will follow, including tire size.. I worked out this post so that you guys can decide on a tire size and it will point to you of what rim specs you need to fit onto the car.. as for brands and types of wheels and tires, well, that's a totally different thread on its own.

-GT's bolt pattern is 5x100mm, NOT 5x114.3. Only the 2005+ Impreza WRX STi is currently running 5x114.3mm. Stock GT wheel is 17"x7" +55mm (still need full verification if it's different from model years), but it's been confirmed for 05' rims on post #1522

-OBXT bolt pattern is 5x100mm with +48mm offset (weighing around 20.5 lbs per rim). OBXT wheel is 17"x7".

-2.5i and 3.0R models will need confirmation on stock rim sizes.

-Center bore must have to be 56mm or larger (if the bore is larger than 56mm, use hub centric rings to fit properly to your wheel's center bore, but not always necessary if you have wheels with conical lugnut seating, the wheel will eventually center itself, but that's your call of how you want to go about that).

-SpecB, OBXT, and 3.0Rs can use shorter tire sizes from the GT. But be sure to check and compare the load capacity of the tires. Try to use the same tire load capacities if you can within +/- 100 lbs. You can certainly always go over, but not under (as this requires for you to run higher tire pressures and this could cause more of an issue for tire wear).

-Lug nuts threading are 12mmx1.25mm. Open or Closed End lugs will depend on the mounting thickness of your rim, usually, closed ends (like the stock ones) are fine.

-Here we go, here are your choices for rim widths (regardless of it being 17", 18", 19" or 20" rims):

{offset in BOLD are the closest possible offset to OEM's scrub radius, you'll notice that once you go past 8" wide rims, you will run out of room on the inboard side to put/stuff any more of the wheel/tire to maintain the OEM scrub radius.. 8" and up will increase positive scrub radius}

6.5" (+34mm to +60mm offset, +55) *for 2.5i only
7.0" (+37mm to +57mm offset, +55)
7.5" (+40mm to +54mm offset, +55)
8.0" (+43mm to +51mm offset, +53)
8.5" (+45mm to +48mm offset, +45) *will need to roll the fenders if tires are wider than 255
9.0" (+43mm to +45mm offset, +43) *will need to roll the fenders if tires are wider than 255
9.5" (+38mm to +40mm offset, +38) *will need to roll the fenders if tires are wider than 255

and it also depends on what kind and size of tire you want to run as well (and that can get messy). But to be on the safe side, here is the tire widths you can put on each rim:

195mm = 6.5", 7" and 7.5"
205mm = 6.5", 7" and 7.5"
215mm = 6.5", 7", 7.5" and 8"
225mm = 7", 7.5", and 8"
235mm = 8", 8.5", and 9"
245mm = 8", 8.5", and 9"
255mm = 8.5", 9", and 9.5"
265mm = 8.5", 9", and 9.5"
275mm = 9" and 9.5"
285mm = 9" and 9.5"

Once you decide on the rim size, the tire width you want to run, here are your final tire choices. Note that not all tires are built the same way, so sometimes you'll find two different tire models with the same size (225/45/17), but one can fit on a 7" rim, while the other one will need a 7.5". You'll have to look around the manufacturers' specs on sectional width of the tire.. normally you want your rim width be no more than 1.5" less than the tire's sectional width. For example, if you have a 8" wide rim, your tire's sectional width shouldn't be more than 9.5" or you'll have to deal with the flex on the tire sidewalls. Most performance driving set ups are 1" difference (ie. a 8" wide rim with 9" wide tire). Below I listed for you by rim diameter of which I recommend running with some notes on each tire with notes to go along with them as well:

16 in. rims (for 2.5i, 2005-on) only:

195/60/16 (great for snow rally or snow use)
205/55/16 (OEM sizing which is also good for some non-tarmac surfaces)
215/55/16 (oversized for the rim protection, but very sloppy sidewall handling)
215/50/16 (wider footprint used for more daily driving without harsh gas consumption)
225/50/16 (great for autocrossing, still usable for daily driving all year round)


17 in. rims (for GTs 2005-on):

205/40/17 (only for SUPER FAST acceleration and torque; for autocrossing only, do not use for daily driving)
205/45/17 (only for FAST acceleration and torque; for autocrossing only , do not use for daily driving)
205/50/17 (good for snow rally)
215/45/17 (stock)
225/45/17 (good autocross performance tire size for gearing, more mph per gear)
235/40/17 (good autocross performance tire size for torque, more acceleration)
235/45/17 (tallest tire you could use under the GT due to possible suspension clearance issues, best used with an 8" wide wheel)
245/40/17 (good autocross performance tire size for traction, will need to have a flatter alignment to make use of the grip)
255/40/17 (maximum grip, good for track days, will need a 9" rim to make use of the contact patch and a good alignment as well).

18 in. rims (for GT 2005-on):

215/35/18 (only good for autocrossing use, do not use for daily driving)
225/35/18 (only good for autocrossing use, do not use for daily driving)
215/40/18 (uncommon tire size, close to OEM overall tire diameter, good for daily driving)
225/40/18 (typical sizing, most popular size, GREAT for daily driving)
235/40/18 (very large, but great for pot-hole city driving, may need fender rolling)
235/35/18 (rare size, but closest to OE tire sizing to keep the spedometer reading correctly)
245/35/18 (common autocross, performance size, very good for daily driving)
255/35/18 (very good daily driving size, even better on the track)
265/35/18 (very good daily driving use, but the car wont be able to use all 265mm of tread for the track)
275/30/18 (tallest tire you can mount without fender clearing issues on a 2" lowered car if you get the right offset)
285/30/18 (aka, the "STREETS OF DREAM" setup... the widest you can go.. this is the next tire size I am going for and a rim to match it)

18 in. rims (for Spec B.2006-on, 3.0Rs 2008-on):

225/40/18 (common tire to use and gives a better acceleration since it's 0.5" shorter)
235/40/18 (rare size to find, but near stock height for proper gearing)
245/40/18 (popular tire size for the larger cars)

19 in. rims (for GT 2005-on):

225/35/19 (common typical sizing for 19" for our cars)
235/35/19 (tallest tire you can mount without fender clearing issues on a 2" lowered car if you get the right offset)
245/30/19 (rare size, but also tallest tire you can mount without fender clearing issues on a 2" lowered car)

Use these at your own risk, but to my knowledge on a lowered car with a small-spring perch type coilover, it should work and ride fine. The most you'll probably have to do to the fenders is to just roll the lip of it in to give yourself another 5mm of clearance. I hope this helps.

I think this is one of the best posts ever! Thanks a lot for all that information, Keefe!

Also, I would sticky this, but I don't have the power to at the moment...

wow yeah thanks!

well, whoever does, definitely put it as a sticky because I am sure someone will want to use it.

Keefe

Thanks Keefe! Great post. I have no more tire/rim questions!

tom

Your Post was really great. All the info I was looking for. I am thinking of getting an 05 GT Limitied.

I want to figure out the best tire/rim size for me.

I am nervious going with a <45 tire since they don't take pot holes and road debrie well. Also, it will roughen up the ride.

What I want to do is get a handeling increase by increasing the tire patch, not by having a narrower tire height.

I think this would mean using a 17" x 8 rim with a 245/40 tire.

My question is does the 245/40 tire have a similar side wall height as the stock 215/45?

Also, what do you think of the daily drivability of the 245/40/17?

Originally Posted by iyamdman

Your Post was really great. All the info I was looking for. I am thinking of getting an 05 GT Limitied.

I want to figure out the best tire/rim size for me.

I am nervious going with a <45 tire since they don't take pot holes and road debrie well. Also, it will roughen up the ride.

What I want to do is get a handeling increase by increasing the tire patch, not by having a narrower tire height.

I think this would mean using a 17" x 8 rim with a 245/40 tire.

My question is does the 245/40 tire have a similar side wall height as the stock 215/45?

Also, what do you think of the daily drivability of the 245/40/17?

Don't be afraid of a 35 series profile tire. I run a 255/35/18 tire and I use it for everyday driving. "Yikes! A 35 series?!" Hold on.. I will show you how a 255/35/18 will actually have the same, if not, more sidewall than a say a 215/40/18. The whole matter is that the series profile is nothing more than a percentage based on the width of the tire.. so the wider the tires, the taller the sidewall (it's all proportional). So the first number is the width in Millimeters, the second number is the percentage of the width of the tire for the profile height, and the third number in the tire size is the rim size. Anyway, here's a simple calculation for the two tires:

255mm x 35%(series) = 89.25mm profile height of the tire
215mm x 40%(series) = 86.00mm profile height of the tire

So you see, just because it's a 35 series doesnt mean it's going to be a thin tire, in fact from this comparison, it's actually thicker. The trick is to find a tire that can fit not only your rims, but under your fenders (if you plan to go that wide).

So for your question "does the 245/40 tire have a similar side wall height as the stock 215/45?". Well, here we go:

245mm x 40% = 98mm profile height of the tire
215mm x 45% = 96.75mm profile height of the tire

So as you can see, it's actually bigger - not by much, but it counts, I can go into the effects of overall tire diameters later.

Today's lower profile tires have better technology such as stiffer sidewalls, reinforced sidewalls, and even run-flats. These are all the matters of keeping your rims from bending. One that I do caution people is that you NEED to check tire pressures frequently to keep them properly inflated so that the tire can support the weight of the car and the abuse of potholes. I will have to put up a new post about "how to properly inflate a set of aftermarket tires."

If you want to go with better handling, you can still do a 225/45/17. It's less weight (comparing to a 245/40/17), your car's stock alignment will still be in compliance and there's a lot of great tire choices in this size. Having a really good performance spec alignment will make use of all 225mm. If you want to have a wider size, you can run a 245/40/17 and run a flatter (less camber) alignment to make use of the acceleration and braking traction. Both are good tire choices (see my side notes on each tire size). The 225/45/17 with an 8" wide rim will definitely be performance driving since you will be able to feel the feedback as you need from the car.

For 18" rims, a 225/40/18 is very doable for daily driving (yes, even for potholes, provided that you drive a little slower over them when you see them and keep your tires properly inflated). For more of a flatter setup and less camber, 245/35/18 is a good choice as well. I run a 255/35/18 on a 18"x8" rim because, well, I am just being Xenonk and I am crazy like that.

As for the rim fitment, that's up to you.. if you plan on running a 245mm tire, run a 8" wide rim. If it's a 225mm, you can run a 7.5". I am going to update my wording on the rim to tire fitment issue to express my thoughts on what works for performance, and what just works for slower-non-performance daily driving.

Hope that clears things up.

Keefe

Hi Xenonk,

You are awsome. This info is the type of geek shiznit that I love. Took this information and played with it in Excel. I calculated the sidewall height for the various series and the tire width from 205 to 265.

Then to add insult to injury, I calculated the wheel/tire circumference for all the combination for the rim sizes from 15" to 20".

Hope the attached file will answer any question about sidewall height, and how various Rim/Series/Width will effect you odometer/speedometer.

U7

As you can see, the Stock Legacy i has 205/55/16, with has a sidewall height of 112.75mm and a tire circumferance of 1985mm


The Legacy GT has 215/45/17, with has a sidewall height of 96.75mm and a tire circumferance of 1964mm.


The difference between the two circumferences is 21mm - the non-GT Legacy move forward 21mm further than the GT with on wheel revolution.

Does this mean the Odometer/Speedometer calculation is different between the two cars, or is the difference so small it is not noticable?


If the later, how much difference matters?


Let's see, 1 mile = 1.6km which is 1600 meters which is 160,000cm which is 1,600,000mm (God, I luv the metric system - it is so easy!)


21mm is .00001% of 1 mile. That would mean you lose 1 mile every 10,000 miles you drive - I think.


Am I off on this?


U7

Awesome, this will help a lot.

Originally Posted by iyamdman

[font=Arial]As you can see, the Stock Legacy...

Just look up the tire datasheets and see the quoted revolutions per mile???? Different models/brands of the same size can actually vary..

Originally Posted by iyamdman

[font=Arial]The difference between the two circumferences is 21mm - the non-GT Legacy move forward 21mm further than the GT with on wheel revolution.

Does this mean the Odometer/Speedometer calculation is different between the two cars, or is the difference so small it is not noticable?


If the later, how much difference matters?
Am I off on this?


U7

21mm can be fixed with just tire pressure and weight of the car. The thing to consider is about going beyond the +/- 2% difference for overall tire diameter.. first thing is, you can't cram in a 30" tall tire in a 25" diameter fender wheel cutout. I gave specific specs about keeping the tires from 24.4" to 25.3" overall tire diameter lengths. I did this because every tire is built in different dimensions eventhough they are all the same size. Example: A 225/45/17 Falken Azenis RT-215 is by far one of the biggest 225/45/17s ever built while a 225/45/17 Bridgestone Potenza RE92 is built not only narrow, but smaller than what a true 225/45/17 would be. I made my post for you guy so that:

A) It wont screw up your odometer/speedometer that much where it can get you in trouble with the law (driving too fast and trying to say "officer, my speedo is broken, I dont know how fast I was going").

B) that the sizes I specify should fit and roll correctly even with modified stock suspension.

C) so that you dont need to do your own calculations and give yourself the headache of finding out what works or what doesnt. I already did the work for you.

If there are any concerns, just PM me, I'll be glad and more than welcome to answer them.

Keefe

Can anyone tell me if the sizing and offset information that Xenonk provided at the start of this thread is applicable to the Outback XT as well? I plan on getting aftermarket wheels for my XT as soon when it arrives later this month and using the stock wheels for snow tires. Given the cost of the wheels I'd rather not make an expensive mistake. If not, can anyone point me in the right direction for the information?

The Outback uses a different set of tires and tire sizing.. but as for the offset and rim size, it should be ok.. just dont use the above tire sizes for your XT (GT tires are smaller in overall tire diameter than the XT). I will have to come up with some new tire sizing for you.. could you let me know what size rim you want to run? and let me know what kind of use you plan for these rims and tires? And to make my life easier, could you give me your stock tire size? Thanks

Keefe

Outback wheels are 17x7 with a +48mm offset. Tires are 225/55-17.

Craig