Spare Tire Requirements: Size, Load, and Speed Limits

Types of spare tires

Spare tires fall into three main categories: full-size spares that match the vehicle's regular tires, T-type temporary spares that are narrow and lightweight, and non-pneumatic spares that do not rely on air pressure. Federal standards cover each type separately. T-type spares are regulated under FMVSS 109, which specifies their load ratings and performance requirements. Non-pneumatic spares fall under FMVSS 129. Full-size spares are simply regular tires and must meet the same standards as the other four tires on your vehicle.

When your vehicle came with a temporary spare, it is often a T-type with a size designation like T125/70D15. These are designed for short-distance use and usually have lower speed and load capabilities than standard tires. If you are considering a full-size spare, you need to verify that it matches the other tires in size, load rating, and speed rating.

Size and diameter matching

The most important fitment check is outer diameter. A spare with a significantly different diameter can affect speedometer accuracy, anti-lock brake operation, and drivetrain components, especially on all-wheel-drive vehicles. To compare, look at the tire size code on the sidewall, such as 225/55R17. The first number is section width in millimeters, the second is aspect ratio (sidewall height as a percentage of width), and the third is rim diameter in inches. An easy way to estimate diameter is to use a tire size calculator or compare revolution per mile figures.

For a full-size spare, the goal is an identical diameter to the other four tires. Some vehicles allow a slight difference, but it is safest to match exactly. If you are replacing a temporary spare with a full-size one, use the same tire size that is on the other wheels. For guidance on reading tire sizes, see our tire size explained article. Also note that a larger or smaller diameter will change your speedometer reading, as explained in tire diameter and speedometer accuracy.

Load rating

Every tire, including a spare, has a maximum load rating printed on its sidewall. Federal regulation 49 CFR 571.110 requires that the vehicle maximum load on the tire must not exceed that rating. It also states that the normal load on the tire must not exceed 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure. For a spare, this means it must be able to support the weight your vehicle can place on that wheel, especially if the spare is installed on a heavily loaded axle.

Check the load index number in the service description, such as 91V. A higher index means a higher load capacity. If you are using a temporary spare, its load rating is often lower than the original tire, so you should reduce the vehicle's load and avoid carrying heavy cargo. For details on load indexes, see tire load index rating. If you need a full-size spare for a vehicle that frequently carries heavy loads, consider an LT-metric tire with a higher load range, as covered in tire load range for light trucks.

Speed limitations

Temporary spare tires, both T-type and non-pneumatic, are designed for limited speeds. The tire sidewall and the vehicle owner's manual will indicate a maximum speed, often around 50 mph but higher on some newer models. Never exceed that limit. Temporary spares are also meant for limited distances, typically just enough to reach a repair shop. Driving long distances on a temporary spare can cause it to overheat and fail.

For full-size spares, the speed rating should match or exceed the vehicle's other tires. If you use a higher-speed-rated tire as a spare, it is fine, but a lower rating may need to be driven more conservatively. The speed rating is part of the service description, like the 'V' in 91V. See our tire speed rating guide for more. Also, note that UTQG grading does not apply to temporary spare tires, according to 49 CFR 575.104, so you cannot compare treadwear or traction grades for these.

Inspection and maintenance

A spare tire needs regular checks even if it is never used. Check the inflation pressure monthly, using the pressure recommended on the vehicle's placard or owner's manual, not the maximum pressure on the tire sidewall. The recommended pressure for a temporary spare is often higher than for regular tires, because of its smaller size. Underinflated tires can overheat and fail, as noted by the U.S. Department of Energy's fueleconomy.gov site, which advises keeping tires properly inflated for safety and efficiency.

Inspect the spare for cracks, bulges, or tread wear. Also look at the DOT date code to see how old the tire is; tires older than six to ten years may need replacement even if the tread looks fine. See tire DOT date code and tire pressure basics for more. If your vehicle has a TPMS, remember that some systems monitor the spare; consult your owner's manual to understand how it works.

Temporary vs. full-size spare

Full-size spares offer the benefit of matching the vehicle's other tires in size, load, and speed rating, so you can drive normally after a flat. They are ideal for long trips, off-road use, or vehicles with AWD/4WD systems where diameter differences can cause driveline damage. Temporary spares save weight and trunk space but impose restrictions on speed and distance.

If you decide to carry a full-size spare, make sure it is the correct size and load rating for your vehicle. For passenger cars and SUVs, see our best car and SUV tires guide. For trucks and heavier vehicles, consider best light truck tires. Off-road enthusiasts might want a matching all-terrain tire. If you have AWD, the spare must be within the manufacturer's specified diameter tolerance to avoid damaging the differential, as explained in AWD tire maintenance.

What to pick for your spare

Questions

Can I use a spare tire with a different size from my other tires?

No. A temporary spare is designed for short-term use at low speeds, but its diameter may be smaller. Using it for more than a few miles can damage your drivetrain, especially on AWD vehicles. For a full-size spare, the diameter must match the other tires to avoid speedometer errors and mechanical stress.

How long can I drive on a T-type temporary spare?

Only as far as needed to reach a repair facility, typically up to 50 miles. The tire sidewall and owner's manual will give specific limits. Exceeding those can cause overheating and failure. Replace the temporary spare with a repaired or new tire as soon as possible.

Should the spare tire pressure be higher than normal tires?

Often yes. The recommended pressure for a temporary spare is usually higher to compensate for its smaller size and to maintain load capacity. Check your vehicle's placard or owner's manual for the correct pressure. Do not use the maximum pressure printed on the tire sidewall.

Can I put a spare tire on the driven axle of an AWD car?

It is best to use a spare that matches the other tires on that axle. If you must use a temporary spare, place it on the rear axle and follow the manufacturer's instructions. Even then, drive slowly and avoid engaging AWD systems until the tire is replaced.

Does UTQG rating apply to temporary spare tires?

No. According to 49 CFR 575.104, temporary spare tires are exempt from the Uniform Tire Quality Grading standards. That means you cannot compare treadwear, traction, or temperature grades for these tires.

What does the letter 'T' mean in a spare tire size like T125/70D15?

The 'T' stands for temporary. It alerts you that this tire is not designed for full-speed, long-distance use. Always check the load rating and speed limit before driving on it.