Mounting your vehicle’s spare tire – which is alternatively referred to as both the donut spare and the space-saver spare – in the event of a flat tire is a common occurrence. The small spare is not intended as a long-term replacement, however. So how long can you drive on a spare tire? The answer may not surprise you, but it could keep you from being a daredevil on tiny tires.
Spare tires don’t have the steel and polyester
Automotive manufacturers realize spare tires are used infrequently, so they’re generally made small to save space and weight. Some vehicles don’t even include a spare. While a spare tire can be a life-saver when you’re facing a flat and have to make it somewhere on time, if you drive on it for longer than the manufacturer recommends, you can run into trouble.
What kind of trouble are we talking about, here? Well, first of all, a spare tire isn’t built to be as durable as a normal, full-size tire. The plies of steel and polyester underneath the rubber tread are what make a tire strong, and spare tires don’t use as many plies. Typically, only one layer of polyester is used in the sidewall, with two steel belts with polyester in the tread. That’s only half as much as what you’d find in a standard, full-size tire. The end result is that both puncture resistance and cornering ability are severely lessened.
Decreased cornering due to small contact
The thinner contact patch a small spare tire presents to the road reduces the overall amount of traction. This makes handling and cornering less sure and even increases stopping distance. Even with ABS and traction control systems in a modern vehicle, spare tires make movement dicey. Smaller tires even reduce ground clearance, making it all but impossible to tow a trailer safety, let alone drive over less-than-flat road. Damage to a vehicle’s differential because of insufficient ground clearance can be endangering to the driver and passengers, as well as expensive.
When it comes to differential problems, Popular Mechanics suggests thinking of it this way: When a vehicle is moving in a straight line, the differential isn’t in use. However, when a vehicle can’t travel in a perfectly straight line – such as when one tire is drastically smaller than the others – the smaller spare tire must spin faster to keep up with the tire opposite it on the same axle. It’s like the car is in a constant turn, which accelerates wear and tear on the differential, gears and clutch plates. This is why manufacturers suggest driving below 50 mph when a small spare tire is involved. This precludes safe freeway driving.
Use the small spare tire if you must
A small spare tire is only intended to be used for a brief period of time. It should be used just long enough to have the damaged tire patched or for a new full-size tire to be purchased. Whether you carry a small spare or a full-size tire as a spare, it is important that you periodically check to ensure proper inflation.