Until every car on U.S. roads uses airless or puncture-proof tires, getting a flat will be something many drivers will continue to deal with. You might be lucky enough to have a full-sized spare, which puts you right back on the road as if nothing happened. Some older cars, though, will have a donut wheel. These rims tout smaller tires than your model’s spec, meant to just get you to a nearby repair facility. But how far can you actually go on one? Use the 50/50 rule to easily remember.

With “50/50” in mind after installing a donut wheel, keep your speed under 50 mph and only plan on using one for about 50 miles. Any more than that, and you risk a couple of things:

• Damaging the donut
• Damaging your car’s drivetrain

Donut wheels are compact enough to fit in a vehicle’s trunk without taking up much space. As such, they aren’t the same size as permanent options. They’re also rated for fewer miles. Driving on them for more than 50 miles, and you’ll risk another blowout.

Moreover, since a donut wheel is much smaller than standard spec, the presence of a donut on one side and a full-size tire on the other creates a “stressful” imbalance that can damage the transmission and prematurely wear steering and suspension components.

By the way, many newer cars don’t have a spare wheel at all. Check your car to see if you need to order one, or at least understand whether you have one so you aren’t completely surprised if you find yourself on the side of the road with a flat.

Driving under 50 mph with a donut wheel and keeping it on for less than 50 miles is best.