How close do AWD tires need to match?

[DJSteve]

The front passenger side tire on my 94 Astro awd has sidewall damage and needs to be replaced. The other three tires have quite a bit of life yet (probably not more than 8-10k on them), however they are a couple years old and this particular tire isn't in production anymore. What's my best plan of action?

If I replace just the one tire, how close do I need to stay to the same circumference? I really don't want to have to replace all four. One tire shop I had check it out suggested I replace in pairs, however I don't understand how that gains me much. Is there any sort of mechanism in the transfer case to compensate for uneven front/rear wheel speed? I didn't really think there was, but I could be mistaken. If that's the case replacing two you'd have to put one on each axle to keep the driveshaft speeds even, but then both differentials would be turning constantly--right?

The tires in question are Uniroyal Radial A/S -- P215/75R15. I'm measuring around 7/32 remaining tread depth--don't have a tire gauge but digital caliper was showing about .215" on rears and .205" on fronts.

[orion33]

it would be best to stay with the same size tire.you dont need to buy two.just a simular tread pattern.make sure to balace it and i always put the unmatched tire on the rear.

[Pantaz]

The closer the better, but a minor size difference shouldn't be a big deal. Many tire manufacturers list tire diameter or circumference on their websites. Measure your tire's current size and see if you can find a close match.

I agree with moving the mismatched tire to the rear. The steering is much more sensitive to size differences than the AWD.

[maximus7001]

Just make sure you have the same correct pressure in all 4 tires and you should be fine.

[gtkane]

GM says max 2/32" difference between tires on any AWD or 4WD vehicle.

[88astroLT]

Yes try and keep the thread heights the same. If the tire really needs replacing i would suggest buying a Used tire (there should be some local near you) that has as close as possible thread depth as the other three.

[tedanderson]

I also agree with putting the mismatched tire on the rear. The rear diff is not as sensitive to differences in tire size. I imagine that the front diff is more particular because it has to contend with being pushed by the transfer case as well as the overall inertia of the van.

[Mr_Bowtie]

I just replaced all 4 tires on my Van cause one blew out................figured they only have 15K left on them MAYBE & they are cheaper/easier then having to fix the transfer case.

[maximus7001]

I also agree with putting the mismatched tire on the rear. The rear diff is not as sensitive to differences in tire size. I imagine that the front diff is more particular because it has to contend with being pushed by the transfer case as well as the overall inertia of the van.

Make sure you have an open rear diff. This may destroy a g80 limited slip if the size is too far off.

Nobody takes into account that 90% of all vehicle tires are not properly inflated or checked. This could make more of a difference than different brands of the same size that are properly inflated and checked.

[rich69]

I was told by a tire dealer that it had to be within something like 3/32 of an inch.
Otherwise, the computer can't properly adjust for the difference.
Quite frankly, I'd say he was selling tires.

[redfury]

On a 94, there is no computer involved in the AWD. The idea behind the tires on an AWD being within a certain tolerance is simple, anytime you have one tire rotating faster than another, there is additional strain on the system, whether it's the transfer case because of a front/rear rolling size difference, or the differentials taking the strain from two different sized tires.

Two scenarios come to mind. Put two tires on one axel so that the strain is lessened all around, or put one tire on any axel and have that one tire throw everything off.

Personally, if I wasn't in a position to replace all the tires or even two, I'd look to find a used tire within the same rolling diameter as the rest on the van. It isn't ideal to mix and match tires on any drive axel, but any tire shop can measure the rolling diameter and get you into a tire that will work for the time being...any other way and you are rolling the dice to see what is going to fail first.

Keep it in spec ( 2/32 ) and you should be fine until you can pull the trigger for 4 new ones.