Tire size calculator
How do you compare two tire sizes?
Enter your current and new tire size. TireFitLab instantly shows the difference in overall diameter, sidewall and width, the speedometer error, and whether the swap stays within the safe tolerance.
- Within tolerance · Diameter +1.7%.
- Speedo @ 60: 60 mph → 61.0 mph.
- Sidewall -0.29 in · Width +0.39 in.
How this result is calculated
Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check wheel-arch, suspension or brake clearance.
Methodology- Diameter
- 24.97 in 25.40 in+0.43 in
- Sidewall
- 3.99 in 3.70 in-0.29 in
- Width
- 8.86 in 9.25 in+0.39 in
- Circumference
- 78.45 in 79.80 in+1.7%
- Revs / mile
- 808 794
- Speedo @ 60
- 60 mph 61.0 mph+1.7%
- Current —
225/45R17 - New —
235/40R18 - Grounded to the road: the axle rises +0.21 in.
Result report
225/45 R17 235/40 R18 · Within tolerance
Use this size when shopping: 235/40 R18
Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.
Safe alternative sizes
| Size | Diameter | Speedo @ 100 | Use this |
|---|---|---|---|
−1″ 285/40R16 | +0.0% | +0.0% | |
OE 225/45R17 | +0.0% | +0.0% | |
+1″ 295/30R18 | -0.0% | -0.0% | |
+2″ 305/25R19 | +0.1% | +0.1% |
Worked examples
These rows use the same geometry engine as the calculator, so the numbers match the interactive result above.
| Current size | Candidate size | Diameter delta | Speedometer at 60 mph | Tolerance |
|---|---|---|---|---|
| 225/45R17 | 235/40R18 | +1.7% (+10.9 mm) | 60 mph → 61.0 mph | within |
| 205/55R16 | 225/45R17 | +0.4% (+2.4 mm) | 60 mph → 60.2 mph | within |
| 215/65R16 | 225/60R17 | +2.3% (+15.9 mm) | 60 mph → 61.4 mph | within |
What to do with it: Use the exact vehicle placard size before buying.
Placard-first
Start from the size already approved for the car
Enter the tyre size from the door placard, fuel flap or current sidewall, then compare a candidate replacement size.Seasonal check
Planning a long summer drive?
Use the budget and running-cost tools before a trip, especially if the current tyres are worn or the replacement size changes diameter.
Popular sizes
- 165/70 R14
- 175/65 R14
- 185/60 R14
- 185/65 R14
- 195/70 R14
- 175/65 R15
- 185/55 R15
- 185/65 R15
- 195/55 R15
- 195/60 R15
- 195/65 R15
- 205/60 R15
- 205/65 R15
- 205/70 R15
- 215/70 R15
- 225/70 R15
- 195/55 R16
- 195/60 R16
- 205/50 R16
- 205/55 R16
- 205/60 R16
- 215/55 R16
- 215/60 R16
- 215/65 R16
More tools
- Tire size chart
- Speedometer error calculator
- Wheel offset (ET) decoder
- Compare sizes
- Load index chart
- Speed rating chart
- Bolt pattern guide
- Rim diameter guide
- Plus sizing guide
- Tire type guide
- How to read a tire size
- Methodology
What changed
- Reviewed deterministic geometry, load/speed references, sitemap inclusion and localized page shell.