Replacing two tires

[subiesailor]

Originally Posted by mikelipke

Tough call. If you replace all 4, you would likely want to go to a better tire, but of course, the 2 being replaced under warranty would be the same crap that came with the car.
Some tire shops can "shave" the new tires to make them the same size as the other 2. Might be your best option.

We just replaced our nearly bald continentals on our 2010 at 40,000 miles. I replaced them with the BFG Touring tires which have a stiffer side wall and a some what similar tread pattern. I have about 500 miles on them now and it is the single best thing I've done to this car the original Continentals would wobble and get out of shape from a light curb touch when curbing the tires in SF where they still write tickets for not curbing your tires. It would take 200 or so miles before the old continentals would stop shaking from the curbing.

The new tires are LIGHT YEARS BETTER!!! Should have done it sooner.
Far better built tires than the conti's - I wouldn't let anyone put another set of those on my car. Miserable tires. I had one that was out of round and could only stay on the back after 25K. Just lousy tires get a better quality tire and you'll be very happy!!!

[plain OM]

If my calculations are close, based on a 27.6" tire, 1/2 inch in circumference difference is equal to 2.5/32 of tread depth.

We can look at this different ways. Here's my approach:

According to cars101.com, the 2010 H6 came with 225/60 R17 Conti ProContact tires.

Continental's web-site specs for the tire show it turns 767 revolutions per mile. That means the tire's circumference is effectively 82.607 inches. Convert this to radius and we get 13.147 inches.

Now, assume another of the same tire that is 1/4-inch smaller in circumference. That's 82.357 inches, which converts to a radius of 13.107 inches.

All other parameters being equal, a difference in tread depth would be a difference in radius. The difference in radius in this case is 0.040 inches, or 1.28/32 (say 1.3/32). In other words, a difference in tread depth of just under 1.3/32 will mean a 1/4-inch difference in circumference.

(Incidentally, I did the same calculations for the 225/55/17 Potenzas on my car and it's the same. A small difference in the overall dimensions of the tire, which will happen between brands and models even when the nominal size is the same, doesn't significantly affect how a small tread depth difference affects circumference.)

You're right in your calculations: a 2.5/32-inch difference in tread depth, which is the difference in radius, will cause a 1/2-inch difference in circumference. However, the recommended difference in circumference should not exceed 1/4-inch, so I'm not sure of the significance of the 1/2-inch difference.

Measuring tire tread depth in 32nds of an inch is not difficult -- standard tire depth gauges, available for a few dollars, have scales calibrated in 32nds, and the readings can be read to 0.5/32.

To measure tire circumference, a flat, preferably metal, tape measure should be used so that it can be drawn around the tire snugly and provide repeatable measurements. (Many woodworking tool suppliers have these. The tape has to be long enough to go around the tire easily.) Distinguishing a 1/4-inch difference should not be a problem -- many tape measures are marked in 16ths, if not finer.

Note: This isn't saying that the 1/4-inch recommendation that Subaru has set out (see link in post 18) has to be followed, for example, to maintain warranty, or that going beyond it will definitely damage the drive train, although both might be true. What you do is your decision, but try to make it an informed decision.

[Green LTD]

I lose you in the details. C = d x pi. The difference between 2.5/32 and 2/32 tread depth ( the oft quoted max) relates to a difference of 1/64 in circumference. I have not seen many rulers that go to 1/64, much less tape measures. Also, I find a spec of 753 revs per mile for this tire, but that makes very little difference in the calculation. Regardless, measuring a tire for a difference of 1/64 would be difficult.

[baobab]

Originally Posted by plain OM

On a 17 inch tire in the size ranges installed on recent Subarus, a 1/4-inch difference in circumference between tires would result from less than a 2/32-inch difference in tread depth.

Measuring tread depth for the purpose of checking for the circumference dimension requirement is only an approximation. If two tires (same brand, model, size, and inflation have tread depths that differ by almost 2/32, there's a pretty good chance they're circumferences will be different by more than 1/4-inch.

Originally Posted by Green LTD

If my calculations are close, based on a 27.6" tire, 1/2 inch in circumference difference is equal to 2.5/32 of tread depth. this is 1/64" or .0156" over the always given 2/32". If you are under the 1/2 inch measured circumference you are very close. (Most tape measures are not going to be able to measure it!)

I do not know why you guys care about the wheel size or tire radius. For two identical tires, a given difference in thread depth will always result in the same difference in circumference, regardless of tire size.

circumference = 2πR
For a thread depth difference of d, you have a difference in circumference of 2πd.

Edit: I see GreenLTD posted a similar thing while I was trying to figure out how to post the symbol for PI.

[Green LTD]

All that being said, my car had 2 tires that were worn poorly at 31,000 miles. Rather than mess around, I went with a new set of Hankook h727's and like them well so far. Have to agree with Subiesailor on this one, probably the best thing I have done for the car.

[ChromeDomeAl]

arizonaadam, when I worked at a tire shop, the official tread depth we used was 2 mm to tell if we needed all new tires. In other words, if the tires will be more than 2mm tread depth difference, you should replace all 4, not just two.

[plain OM]

Originally Posted by baobab

I do not know why you guys care about the wheel size or tire radius. For two identical tires, a given difference in thread depth will always result in the same difference in circumference, regardless of tire size.

circumference = 2πR
For a thread depth difference of d, you have a difference in circumference of 2πd.

Agreed. Whether it's calculated my long way, or just take the difference as you suggest, it still means that a difference of less than 2/32 in tread depth will result in a 1/4-inch difference in circumference.

Originally Posted by Green LTD

The difference between 2.5/32 and 2/32 tread depth ( the oft quoted max) relates to a difference of 1/64 in circumference. I have not seen many rulers that go to 1/64, much less tape measures. Also, I find a spec of 753 revs per mile for this tire, but that makes very little difference in the calculation. Regardless, measuring a tire for a difference of 1/64 would be difficult.

Not sure where the "difference between 2.5/32 and 2/32 tread depth" comes in. The issue as I see it, and as reflected in the linked Subaru TechTips article, is that the tires all have the same circumference within about 1/4 inch. Addressing the concern that measuring circumference might be difficult, then the question is: what difference in tread depth on otherwise identical tires would end up leaving us with a 1/4 inch difference in circumference?

Taking the formula approach that baobab suggested, and inverting it, we get:

d=difference in circumference/(2*pi), where the difference in circumference = 0.25 inches. This calculates to 1.27/32-inch. In other words, a difference of less than 2/32-inch will result in a circumference difference of 1/4-inch.

A difference in tread depth of 0.5/32 (the difference between "2.5/32 and 2/32 tread depth") will indeed result in a far smaller difference in circumference than 1/4 inch (actually, it comes out to 6.3/64), but it seems to me that, according to Subaru, we need not concern ourselves about a 0.5/32 difference in tread depth.

As I noted earlier, the 2/32-inch figure is not cited in Subaru sources; but it might be easier to measure tread depth than circumference, and 2/32 perhaps serves as a quick, measurable-on-the-car, outside limit for Subaru's "approximately ¼" of each other in circumference".

As the issue is (or should be) referenced to the 1/4-inch in circumference, there's no need for "measuring a tire for a difference of 1/64". A tape measure that's marked in 1/16-inch increments (4/16=1/4-inch) would be quite adequate.

[Green LTD]

The manual states 2/32 on gen 4 Outbacks. At issue perhaps should be the published spec. And so if one tire is inflated to 32 lbs and another only has 22 lbs, what is the difference in rolling circumference? Being a little snarky here, but the Gen 4 does not have any viscous differentials, manual tranny excepted, as the specs you quote are clearly intended for.

[GasGuzz]

My question, is psi is a determining factor.
The natural condition of a tire is when it’s rolling (when the suspension meets equilibrium), that’s why it’s balanced free/unloaded and at speed. The tire’s belt construction maintains its structure (dimension/circumference), while psi complements its loading parameters/limits. In fact/example, the OB is spec’d with different F/R psi, on same tires (higher in F for braking/loading).

[Peaty]

http://www.scoobymods.com/showthread...33.html?t=3433