Spare Tire Size Comparator
Compare your spare tire's diameter with the road tires. Check compact T-type spares for diameter gap, height drop and speed limits.
Compare spare and road tire dimensions.
Spare Tire Size Comparator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Spare Tire Size Comparator Works?
The Spare Tire Size Comparator works by calculating the spare's diameter and comparing the spare with the road tire on the same axle. The tool uses 3 steps.
Enter the road tire
Type the size on the 4 road wheels, for example 225/45 R17.
Enter the spare
Type the spare's marking, for example T125/70R17.
Read the gap
The comparator shows the diameter gap, the corner height drop and the temporary-use limits printed on T-type spares: 80 km/h (50 mph) and about 80 km (50 mi).
Spare Tire Size Comparator Formula
The Spare Tire Size Comparator formula is Δ% = (OD_spare − OD_road) ÷ OD_road × 100, and the corner drop is (OD_road − OD_spare) ÷ 2.
OD_s = 2 × W × AR ÷ 100 + Rim × 25.4Δ% = (OD_s − OD_r) ÷ OD_r × 100Δh = (OD_r − OD_s) ÷ 2- OD_s
- Spare diameter (mm)
- OD_r
- Road tire diameter (mm)
- Δh
- Height loss at the spare's corner (mm)
Worked example
A T125/70R17 spare measures 606.8 mm against 634.3 mm for a 225/45 R17 road tire. The gap is -4.34%, and the corner sits 13.8 mm (0.54 in) lower.
Spare Tire Size Comparator Conversion Table
The Spare Tire Size Comparator conversion table compares 6 spare options with a 225/45 R17 road tire.
| Spare | Diameter (mm) | Diameter (in) | Gap | Corner drop |
|---|---|---|---|---|
| Full-size matching | 634.3 mm | 24.97 in | +0.00% | 0.0 mm |
| Compact T125/70R17 | 606.8 mm | 23.89 in | -4.34% | 13.8 mm |
| Compact T135/80R16 | 622.4 mm | 24.50 in | -1.88% | 5.9 mm |
| Compact T145/70R18 | 660.2 mm | 25.99 in | +4.08% | 13.0 mm |
| Compact T125/80R16 | 606.4 mm | 23.87 in | -4.40% | 13.9 mm |
| Older full-size 205/55R16 | 631.9 mm | 24.88 in | -0.38% | 1.2 mm |
What is Spare Tire Size Comparator?
A Spare Tire Size Comparator is a tool that compares a spare tire's dimensions with the vehicle's road tires. Compact temporary spares, marked with a T prefix, are narrower and often slightly shorter than road tires.
Compact spares carry a printed speed limit, usually 80 km/h (50 mph). A diameter gap of more than 3% on a drive axle forces the differential to work continuously, so most makers tell owners to fit the compact spare on a non-driven axle.
Owners who upsize road tires need to recheck the spare. A spare that matched stock tires falls further behind a taller aftermarket size.
What are the uses of Spare Tire Size Comparator?
There are 4 main uses of the Spare Tire Size Comparator, including upsizing checks and full-size spare selection.
Upsizing checks
Confirm the old spare still suits taller aftermarket tires.
Full-size spare selection
Pick a full-size spare that matches road tire height.
AWD precautions
Measure the gap that an AWD system sees with a compact spare fitted.
Jack and lift space
Estimate the corner drop when changing a flat.
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