rotate according to tread depth or circumference?

[oilyriser]

I went to rotate the tires on my car, after checking tread depth. The fronts had less tread than the rears, and on a Subaru you have to keep them even or some power is lost from slippage between the two ends of the AWD system. Being anal about this, I took off the tires, and measured around them with a tape measure in two places, just to make sure. To my surprise, the fronts were larger than the rears.

I'm not sure if the tread was moulded differently, if the belts are slightly longer on one pair of tires, or if they are mounted slightly differently on the rims. But I've learned it's a good idea to get out the tape measure for this car, and not just look at the tread.

 

[javacontour]

Were the air pressures the same? I wonder if inflation pressure makes a significant difference in the circumference of an unloaded tire?

 

[PW01]

I just rotate based on mileage, every 5000

 

[oilyriser]

Yes, same pressures. I'd like to get an OBD2 interface so I can read the wheel speed sensors. That would tell for sure the relative circumference of the tires. In a two wheel drive car, it doesn't matter, but the AWD system has slippage at the center differential if the front wheels' total circumference doesn't match the rear, and that uses a tiny bit extra gas.

 

[Tosh]

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I'd like to get an OBD2 interface so I can read the wheel speed sensors.

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Which OBD2 thingy shows this?

 

[Master ACiD]

even a 2wd has slippage between the right and left.

 

[alreadygone]

"Being anal about this, I took off the tires, and measured around them with a tape measure in two places, just to make sure. To my surprise, the fronts were larger than the rears."

Yep, pretty anal allright. To avoid discrepancy, draw a line on hard surface, another line on tire at 6:00 (dead bottom). Roll tire untill it's line is back to 6:00,mark hard surface again. Now measure distance between marks for much more accurate measurement. Or get a string to wrap around tire, then measure string. Either will be more accurate than attempting to wrap metal tape around tire.

Bob

 

[CapriRacer]

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......Being anal about this, I took off the tires, and measured around them with a tape measure in two places, just to make sure. To my surprise, the fronts were larger than the rears.......

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The obvious question is: How much difference was there?

I would think that anything within 1/2" would be OK, but this is something that the vehicle manufacturer would have to to tell you. My impression is that "they" don't supply this type of info - just some vague "the tires have to be the same size, type, etc."

 

[George Bynum]

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Quote:

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......Being anal about this, I took off the tires, and measured around them with a tape measure in two places, just to make sure. To my surprise, the fronts were larger than the rears.......

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The obvious question is: How much difference was there?

I would think that anything within 1/2" would be OK, but this is something that the vehicle manufacturer would have to to tell you. My impression is that "they" don't supply this type of info - just some vague "the tires have to be the same size, type, etc."

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CR, wasn't it you who discussed the (relatively fixed) belt length having more influence in revolutions per mile than the squirming rubber? If so, and I think it is, same brand/style/size would be really close in revolutions per distance whether new or not new.

 

[moribundman]

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Quote:

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......Being anal about this, I took off the tires, and measured around them with a tape measure in two places, just to make sure. To my surprise, the fronts were larger than the rears.......

---

The obvious question is: How much difference was there?

I would think that anything within 1/2" would be OK, but this is something that the vehicle manufacturer would have to to tell you. My impression is that "they" don't supply this type of info - just some vague "the tires have to be the same size, type, etc."

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I don't know about Subaru, but Audi says the difference in tire circumference between front and rear should not exceed 3% on a quattro. A larger difference can supposedly cause additional wear somewhere in the drive unit (Possibly in the torsen? I don't remember). I've never actually measured the difference and eyeballed it instead, going by tread depth. I have not run into any issues over 160k miles.

 

[Eddie]

I am a Subaru enthusiast and Subaru wants NO MORE than 1/4 inch difference in circumference between front and rear tires. My Foresters 215/60/16 tires reach the 1/4 inch difference at ~ 2/32 in difference in tread depth. The 1/4 inch difference is on an unloaded tire and yes the tire pressure and weight distribution effect this but, the default transfer gear ration is 60/40 so that compensates for the pressure & load distribution.

 

[Swift]

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I just rotate based on mileage, every 5000

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Yup

 

[CapriRacer]

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....CR, wasn't it you who discussed the (relatively fixed) belt length having more influence in revolutions per mile than the squirming rubber? If so, and I think it is, same brand/style/size would be really close in revolutions per distance whether new or not new......

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Yes, it was me, but the operative word here is "relatively". According to Eddie, Subaru say 1/4" in circumference! Whew - that's pretty tiny!!!

Mori says Audi says 3% - that's 2 1/2" difference in circumference.

I ran across a situation where are Volvo had repeated problems with tires that were 1 1/2" difference in circumference.

That's quite a difference between vehicles.

But, I'd still like to know how much actual difference there was.

 

[TheTanSedan]

On all vehicles I use time/mileage first. Then tread depth. Anything over 40,000 miles is gravy, IMHO, as tire wear internally is about impossible to measure. Even "expensive" tires are cheap in comparsion to loss of one while driving.

 

[Lorenzo]

Oilyriser, is this a manual or automatic transmission?

Subarus with automatics actually have a 90% front / 10% rear torque split under normal conditions. This would account for greater wear on the fronts.

You didn't say how many miles since the last rotation, but perhaps it should be more frequent.