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Old 09-19-2009, 01:31 PM   #1
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Question Tire size differences...how much is acceptable for ABS and differential?

This is for my 2002 GMC Sierra 1500 4x4.

I have acquired one new and four used tires specified at the same size as eachother. All are 265/70-16. Two are the same model as the one new one (BFG Rugged Trail) and two are Dunlop Grantrek AT21.

I rolled them out and these are the results for the 5 tires I have, measured in inches of circumference and calculated to Revolutions Per Mile:
- 1 @ 670.476 RPMi (94.5") - the new BFG
- 1 @ 673.326 RPMi (94.1") - BFG
- 1 @ 675.84 RPMi (93.75") - Dunlop
- 2 @ 676.923 RPMi (93.6") - one each of the BFG and Dunlop

Am I overthinking this? Should I just shove 'em on the truck and enjoy having one new tire? What position should I use the new tire in?
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Old 09-19-2009, 05:25 PM   #2
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Are those numbers with the tires inflated, on rims, under the weight of the vehicle?

You really have to change the diameter by about 10% before you start affecting ABS operation.

For a differential, radius should be no more than 2/32" of an inch.

-BC
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Old 09-19-2009, 05:36 PM   #3
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No, those numbers are unmounted (so also uninflated and unloaded). Do you suppose the differences will get better or worse when mounted + inflated + loaded?

2/32" radius = 1/8" diameter, that's very close. With my front diff disconnected from the engine except on very slippery surfaces, does that make it able to tolerate mismatch? Or, even with its driveshaft able to spin free, does a mismatch cause a problem?
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Old 09-19-2009, 09:51 PM   #4
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Well if your car has all open diffs, then I don't think it's really all that bad for your car to have a slight mismatch going on. Even a transfer case will tolerate some front-back difference without too much of a problem. Heck, there are bearings and gears that absorb different rotations all the time with no problems. Of course if there is *too* much preferential rotation, you'll wear the gears in one direction more than the other and after a few hundred thousand on the odometer you might notice something.

If you were driving a car with AWD or some other "high tech" 4WD system, I'd be more worried about the tire diameters. An Audi AWD or Subaru comes to mind. Those things do have a tough time with significant differences in tire diameter.

It's difficult (impossible) to say what will happen to the radii when you have mounted/inflated/loaded the tires, that changes the whole sidewall & tread geometry too much. But in reality, I think you'd be fine.

-BC
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Old 09-20-2009, 03:56 AM   #5
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Yup, both diffs are open.

It's hard to decide how to place them. Maybe I should go ahead and get them mounted, then roll them out again and go with those measurements.
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Old 09-20-2009, 08:24 AM   #6
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What will happen is you will hear a droning sound as the differential slowly rotates the output axles at a slightly different speed like when taking a mild sweeping turn on the highway. If you run the different diameters in front with rear wheel drive it should not be a problem and the only thing to check is to see if it pulls towards the smaller diameter tire when you brake so that being said you probably should put the larger tire on the right front to make up for the crown of the road pulling right already.
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Old 09-20-2009, 10:24 AM   #7
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A droning sound doesn't scare me (unless I forget why it's happening ).

All last winter I only used 4wd when I was on unpacked or smooth slick-packed snow/ice, not on patchy stuff or packed but driveable snow, and I can do it again.

Maybe next year I can afford a set of new tires (maybe even snow tires).
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Old 11-18-2010, 04:03 PM   #8
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Re: Tire size differences...how much is acceptable for ABS and differential?

Good to hear that over 10% is the critical - we will see soon as I just ordered two tires today from TireRack a little bigger than the stock tires for my xB. 185/65R15 are 24.4 and the stock still in the rear tires are 185/60R15 23.7 diameter - After I ordered two from TireRack they called and were concerned abou tthe ABS not liking the difference in size - looks like about 3% larger diameter which is about the right amount for the speedo error woo woo finally will be dead on. If anything the ABS will think the rear tires are skidding and reduce brake power to them. We will see in a week or so when I mount them up.
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Old 11-19-2010, 04:57 PM   #9
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Re: Tire size differences...how much is acceptable for ABS and differential?

Keep me posted on how well those tires work for you. I'm looking to do the same on my box when its tires need replacing.
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Old 11-19-2010, 07:03 PM   #10
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Re: Tire size differences...how much is acceptable for ABS and differential?

Yeah A guy in chicago says they work great in snow so I went a size bigger on the sidewall - they weigh the same 19lbs so no issues there. Should be here on Monday and I will mount them when I can - before the snow comes.
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