Tire size options?

[OutbackGT]

Okay, while I'm waiting to receive word on a set of JDM Bilsteins I've had my eye on, I started thinking about tires. Now, I plan on lowering my car about 2 - 2.5 inches with the Legacy struts and lowering springs, so I'm thinking the stock tires might rub. Ultimately the plan is to get some sexy gold BBSs and high performance rubber, but in the mean time, I think I'll opt for some decent tires with a thinner sidewall. According to Tire Rack, the OEM tire size is:

225/60-16

If I recall correctly, the "60" is the aspect ratio of the sidewall. Would it be safe to go with some 225/50-16 or 225/45-16 tires for a thinner sidewall? Any guidance here is appreciated. Thanks!

[Sarra]

you could certainly go with a lower profile tire. However, if you change the profile of your tires, you'll run into a few problems.

The first is going to be ride comfort. The sidewall of your tires will change the ride comfort a lot. The lower the profile of hte tires, the rougher the ride.

Second is tire choices. I'm sure you've got a ton of tire choices at your stock tire size, 225/60-16 is a popular tire size, iirc. I think you might have trouble finding a tire that is 225/46-16.

Third is going to be your speedo. If you put 225/45-16 tires on your car, your speedo will show a speed that is 10% slower than you're atually going. You will put more miles on your odometer than your car actually travels.

Fourth is going to be ride height. You're going to lose about 2-3 inches of ground clearance by using a smaller sidewall.

Fifth is going to be gas milage. you have a 4EAT, in your H6, unless you swapped it, I'm assuming, so 4th gear is going to be very short for curising on the freeway. If your gearing top speed was 150 with the stock tires, your new gearing top speed will be about 10% lower, 145 ish. On the freeway, your RPM will be higher at the same speeds, you'll use up gasoline a bit faster.

On the flip side, you'll get more acceleration.

I'd also watch out for brake problems, though that seems unlikely.

edit: Go here http://www.rims-n-tires.com/rt_specs.jsp

Put in 225/60-16 | 16/6.5 50 on the left, and 225/45-16 | 16 6.5 on the right. You'll see what I mean.

[Huffer]

this is a good calculator also:
http://www.miata.net/garage/tirecalc.html

[OutbackGT]

Thanks guys. Very informative.

On a somewhat related question, will 99-04 Legacy GT wheels fit my car? They look to be about the same size, is the offset and bolt pattern the same? Are they any lighter?

[Sarra]

The 18 spoke wheels are the exact same size as the H6's wheels, IIRC. Well, do you know if your H6 wheels are 16X6.5 inches or 16X7 inches?

The 18 spoke Legacy GT wheels are 16X6.7, either way. But yes, the 18 spoke Legacy GT wheels will fit on your H6. As for weight, I don't have a clue. I've never ever had a set of 16" wheels until this car, and they're aftermarket. =\ No clue.

[ivwarrior]

Fourth is going to be ride height. You're going to lose about 2-3 inches of ground clearance by using a smaller sidewall.


He's planning on lowering his Outback using Legacy suspension bits. Losing some ride height is the whole point. :)

A stock 2001 Legacy GT comes with 205/55/16. Tire Rack lists optional sizes of 205/50-17 and 215/40-18.

Now, does anyone know what the speedo difference is between the Legacy and the Outback? Do they just use a different gear to drive it, or is it controlled through the ECM and programmed differently? If it's just a differnt gear, getting a Legacy GT gear should solve the speedometer/odometer errors.

According to the Miata tire caclulator, a 205/55/16 and a 225/50/16 are virtually identical in height/circumference, so you can probably get away with the wider 225 series tire.

[OutbackGT]

A stock 2001 Legacy GT comes with 205/55/16. Tire Rack lists optional sizes of 205/50-17 and 215/40-18.


According to the Miata tire caclulator, a 205/55/16 and a 225/50/16 are virtually identical in height/circumference, so you can probably get away with the wider 225 series tire.

Question: How is it possible that one rim can accept tires of differing tread width (205, 215, etc.)? And how is it possible that a 16 inch wheel like the Legacy can accept wheels designated as 17 and 18 inch? As you can tell, I'm a tire n00b.

[shazapple]

He means if you have 17 and 18" rims then thats the size of tire you'd want to go with.

You should see VW owners. Some of them have an insane obsession with putting thin tires onto a wide rim.
http://forums.vwvortex.com/zerothread?id=2960995
http://i26.photobucket.com/albums/c102/dick5238/HRCoilovers003.jpg

Anyway, Im not sure how the bead of the tire holds itself on the rim but it just goes to show how much a sidewall will flex to meet the rim

[AussieDan]

Basically, you have 2 measurements for the rim that directly affect the tire size you need for that rim (we'll ignore offset here for simplicity).

1. diameter, this will be 16, 17, 18 inches etc.
2. width, this is the 7, 6.5, 8 measurement, again in inches.

Your stock rims will be either 16x6.5 or 16x7

Now, the tire measurement has 3 parts.

Section width / Aspect ratio R Rim diameter

eg 205/55R16 means:

205mm section width
55% aspect ratio (this is also called the series)
16 inch rim

The rim size MUST match, you can't fit a 16" tire on a 17" rim, or vice versa.

The tire manufacturer will specify the range of rim width for any given tire. In general on a 6.5" wide rim you'll be looking at a 205mm tire, on a 7" rim you can fit up to a 225mm tire. This depends on the particular tire and the intended use (usually staying in the middle of the range will give you the best compromise between traction, handling and comfort).

The middle number is what tells you the overall height of the tire, in general terms it tells you the sidewall height as a percentage of the section width, so a 205/55 tire has a 205 * .55 = 112.75mm sidewall. If you want to run a wider tire you'll need a lower aspect ratio to maintain the same rolling diameter, eg for a 225 width you'd want a 50 series tire (225 * .5 = 112.5mm) to get the same sidewall height, and therefore the same rolling radius.

I cooked up a little calculator when I was getting my head around this stuff, you can find it here:

http://mr2.phpwerx.net/turbocalc/tiresize.php

[rougeben83]

A stock 2001 Legacy GT comes with 205/55/16. Tire Rack lists optional sizes of 205/50-17 and 215/40-18.


According to the Miata tire caclulator, a 205/55/16 and a 225/50/16 are virtually identical in height/circumference, so you can probably get away with the wider 225 series tire.

Question: How is it possible that one rim can accept tires of differing tread width (205, 215, etc.)? And how is it possible that a 16 inch wheel like the Legacy can accept wheels designated as 17 and 18 inch? As you can tell, I'm a tire n00b.

A tire has a bead that seats on the rim edge when inflated. This is very tight and what holds the air in. A rim width's can accept a range of tire sizes as long as the tire bead is able to seat itself inside the rim.

For different rim diameters, all that matters is the overal circumference of the tire used. You can change the diameter of the rim all you want, as long as you get a tire with the right sidewall height to compensate and retain the overall diameter.

The overall diameter of the tire is what the vehicle's suspension geometry is optimized to perform with, as well as what the car uses to calculate speed and mileage.

[OutbackGT]

Thanks, guys. I think I'm reasonably informed now.