Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

New spin to donut tire question

We agree that diameter is important…how much of a difference in tread depth(or mileage) between a new full size spare and the flat tire’s will result in a differential problem and then, what is the minimum mileage one can drive safely to get the tire repaired? before damage will result.

Please read the owner’s manual for any special considerations when driving on the spare. This was suggested in a similar post.

But, to try to give some insight to the question, it really depends on the car or truck, and the drive system, being 2WD, 4WD, or AWD. These are all different systems. Most 2WD systems will not experience abnormal wear due to slight differences in tread depth. In fact, I had a full sized spare with my Toyota P/U (2WD) with a matching rim to the other tires. The owner’s manual suggested I rotate the spare into use, with one of the other tires moved to the spare with every rotation. Of course, this helped with the wear, but it was always going to force one rolling tire to have different wear than the others.

On 2WD systems, limited slip and posi-traction lock-up differentials will increase wear exponentially with the difference in diameter. I have limited slip in my SUV, and had to use my full-sized spare last year. I used it for 8 miles, 5 to get home, and 3 to the tire store for a new pair. I don’t like using the spare, and I think the steel rim looks ugly next to the factory mags. I suggest if you have limited slip, change the tires in pairs, and rotate with matching pairs on the rear axle. The owner’s manual has no warnings for using the spare, only requirements for gear lube to be used in the differential.

Part-time 4WD is similar, but most people only use the 4WD portion on slippery surfaces, which negates the effect. These systems generally don’t have a center diff, and turning requires at least one axle to slide to turn. I do know the owner’s manual in most of these systems warns against using 4WD on dry pavement, or serious damage can result.

AWD is different. These systems employ 3 differentials typically (front, rear, and center), and most are very sensitive to tire diameter differences. They usually require all 4 tires to be changed together, a proper rotation schedule must be used, and the spare tire is for very short-term emergency use only. Each system has it’s own level of sensitivity, so it is best to read the owner’s manual to determine what to be careful of.

As far as minimum mileage before damage, again depends on the system. The owner’s manual is your best source of information.

Read owners manual…Toyota RAV4 AWD full size spare…no specific information other than do not mix tire sizes and all four must be matched. If one has, say 15,000 miles of road wear and need to use the correctly inflated full sized spare, then how many miles can one drive safely before differential problems result? There will be a diameter difference between the other three tires and the new spare now in use.

I’d use it no longer that absolutely necessary to minimize damage. The fact that all 4 must be matched leads me to believe that an unmatched tire will cause abnormal wear. And, that all 4 tires must be replaced together. If the flat cannot be repaired, it and the other 3 tires must be replaced. Also, be sure to keep an eye on the tire pressures. An under-inflated tire can also cause abnormal wear.

"no longer that absolutely necessary to minimize damage. ’
That advice is a given…how much of a mismatch will cause damage?

For exact figures, you’ll need to contact the engineers at Toyota.

For exact figures, you'll need to contact the engineers at Toyota.

I guessed that when I first thought of the question since there is really no safe answer…I guess I was just doing what I normally do…asking obtuse questions seeking exact answers…thankx.

shop to patch the not to be driven on for a long periods of time.that is why it is called a spare.unsafe to drive on a spare for along period of time

If the flat cannot be repaired, it and the other 3 tires must be replaced.

Not strictly true. If the other 3 tires have a lot of tread left, it may be practical to “shave” down the tread on a new tire to match the others. They grind off some of the tread. Note that this is not the same as “siping” a tire, which is a stupid and unnecessary thing.

Seemed like a really thoughtful intelligent question to me, but then you know me

That is the key. Minimal mileage. The donut won’t have any traction. You don’t want fender damage either. You won’t have differential damage unless you have all wheel drive. Then you will have a spare that is the same size as your other tires. Then you will have differential damage a little later.