Spare Tire Size Comparator
Compare your spare with the tires on the road. Use the calculator below. Results update as you type.
How the Spare Tire Size Comparator Works
This comparator works by computing the diameter of a temporary spare and a road tire and reporting the gap in percent. It shows how much smaller the spare is.
- Enter the spare size, for example T155/70R17 (25.54 in).
- Enter the road tire size, for example 225/60R17 (27.63 in).
- Read the gap: −7.55%.
- Read the speed note printed on the spare, usually 50 mph (80 km/h).
The Formula
The formula is Gap (%) = (Spare Diameter − Road Diameter) ÷ Road Diameter × 100.
Overall Diameter = (Rim × 25.4) + 2 × (Width × Aspect Ratio ÷ 100) Gap (%) = (D_spare − D_road) ÷ D_road × 100The T prefix marks a temporary-use spare. The formula treats the T size like any other metric size. Worked example: T145/80R18 is 27.13 in, and 225/55R18 is 27.74 in, so the gap is −2.20%.
Conversion Table
The conversion table compares 8 temporary spares with 8 road tires. Every spare is smaller, with gaps from −2.20% to −7.55%.
A spare on a driven axle with a large gap makes the differential work all the time. Keep spare use short and slow.
| Spare | Spare Dia (in) | Road Tire | Road Dia (in) | Spare vs Road |
|---|---|---|---|---|
| T125/70R16 | 22.89 | 185/60R15 | 23.74 | -3.58% |
| T125/80R17 | 24.87 | 205/55R17 | 25.88 | -3.88% |
| T135/70R16 | 23.44 | 195/55R16 | 24.44 | -4.11% |
| T135/80R17 | 25.50 | 215/55R17 | 26.31 | -3.07% |
| T145/70R17 | 24.99 | 205/55R17 | 25.88 | -3.42% |
| T155/70R17 | 25.54 | 225/60R17 | 27.63 | -7.55% |
| T145/80R18 | 27.13 | 225/55R18 | 27.74 | -2.20% |
| T165/80R17 | 27.39 | 235/65R17 | 29.03 | -5.63% |
Temporary (T-type) spares are typically limited to 50 mph (80 km/h); follow the limit printed on the spare.
What Is the Spare Tire Size Comparator?
The Spare Tire Size Comparator is a tool that compares a compact temporary spare tire with the road tires on a vehicle. Most compact spares are narrower and shorter than road tires to save weight and trunk space.
The comparator shows the size gap that the ABS, traction control and differentials see while the spare is fitted. A large gap can trigger warning lights and limits AWD operation on some vehicles.
Common Uses
This comparator has 4 uses: emergency planning, AWD checks, used-car checks and spare upgrades.
- Plan emergencies: Know the spare's limits before a flat.
- Check AWD vehicles: See how far the spare sits from road size.
- Inspect used cars: Confirm the spare suits the vehicle.
- Upgrade spares: Compare a full-size spare option.