Five things you should know about a spare tire

You get a flat, and under the carpet in the trunk is a donut spare tire. You know, the temporary spares that are much skinnier than the tires on your car, with a prominent warning about maximum speed on the side. You're probably wondering how long you can drive on it before you get your other tire repaired at a reputable shop. We're here to help.

The best place to start is your owner's manual, which should list the exact recommended mileage for the spare tire. If you don't have a manual, 70 miles is the rule of thumb. Temporary, donut spare tires are not designed for long-range service. They are made to get you to a garage to have your tire either repaired or replaced.

Driving on it for an extended period of time under regular driving conditions will result in a blowout in short order, so get a safe tire on your car immediately. 

How long can I drive on a temporary spare tire?

Donut spare tires are not made for extended service. They have little tread to speak of, the carcass or body of the tire is not reinforced, and there is no belting to protect it from punctures and road imperfections. The tire is designed simply to get you to a repair facility. Most of these tires offer a maximum life of 70 miles.

There's another reason to avoid driving on the spare: mechanical damage. The smaller spare tire can cause premature wear on your differential. Replacing the differential is a lot more expensive than replacing a tire. Trust us. Get the tire repaired as soon as possible, and don't push your luck.

How fast can I drive on a donut spare tire?

It should say right on the side of the spare tire, or on the wheel itself on a large, prominent sticker. If that's missing or illegible, the rule of thumb is to not drive faster than 50 mph with a donut spare tire. Going faster could cause tire failure, differential damage, or both. Try to avoid the freeway if possible. And get to a tire repair facility as soon as possible.

Do temporary spares affect braking or handling?

You may notice poor cornering, handling, and braking characteristics. The donut spare has a smaller footprint than a conventional tire, which translates into diminished braking, roadholding and handling. With the donut spare in place, you may find that, when braking, the vehicle dips to the side where the donut was mounted. Also, you may notice the vehicle pulling to that side.

Why do I have a temporary spare instead of a full-size one?

The use of donut spares is the result of limited space inside the vehicle in question or car makers seeking to reduce cost or weight. Typically the donut spare takes up much less space and is cheaper to manufacture than full size tires and wheels, which translates into more profit for the car makers. It also is significantly lighter than a full-size tire and wheel, which helps with fuel economy. Many cars eliminate spare tires of any sort altogether, and instead include a small tire inflator or can of tire goop, in the name of further weight savings and greater efficiency. Outfitting the car with run-flat tires is another frequent choice. These allow your to drive a comparable distance as the temporary spare would allow even with a full loss of air. The catch is they are much pricier to replace.

For the consumer, anything less than a full-size spare is quite a compromise in the event of a puncture.

Can I use a full-size spare tire instead?

Yes, but the question is: where do you carry and stow it? Years ago, cars had spare tire wells to fit the full size spare so that it was out of the way. This space has been eliminated in order to save weight and space. It looks like we're stuck with donut spares, or no spares at all.

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