Speedometer error guide
What is speedometer error?
Speedometer error occurs when the rolling circumference of your fitted tires differs from the circumference the manufacturer calibrated the speedometer against (the stock tire size). A larger-than-stock tire makes the speedometer read low (you go faster than it shows). A smaller tire makes it read high. The EU directive allows +6 / −0% indicated-vs-actual; most factory setups use +1–3% positive bias.
- Speedometer error occurs when the rolling circumference of your fitted tires differs from the circumference the manufacturer calibrated the speedometer against (the stock tire size).
- A larger-than-stock tire makes the speedometer read low (you go faster than it shows).
- A smaller tire makes it read high.
FAQ
- What is speedometer error?
- Speedometer error occurs when the rolling circumference of your fitted tires differs from the circumference the manufacturer calibrated the speedometer against (the stock tire size). A larger-than-stock tire makes the speedometer read low (you go faster than it shows). A smaller tire makes it read high. The EU directive allows +6 / −0% indicated-vs-actual; most factory setups use +1–3% positive bias.
- What should I verify before using this information?
- Use TireFitLab values as a sizing reference, then verify the vehicle handbook, tire placard, rim compatibility, load rating, and physical clearance before fitting.
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.
How speedometer error is calculated
The error percentage is based on the change in rolling circumference (= overall diameter × π):
Error % = (new OD − stock OD) / stock OD × 100
A positive result means the speedometer reads low (car goes faster than shown). A negative result means the speedometer reads high (car is slower than the indication).
Legal limits by region
| Region | Directive | Limit | Note |
|---|---|---|---|
| European Union | UNECE R39 / EU 2009/40/EC | Indicated ≥ actual AND ≤ actual + 10% | Speedometer must never read below actual speed |
| United States | FMVSS 101 | Within ±5 mph of actual (US cars) | Less strict; most US factory setups also run +1–3% |
| General recommendation | TireFitLab calc | +2 % / −0 % | Our safe-substitution default (within both US and EU limits) |
Example errors (stock: 205/55R16)
| Alternative size | Error | At indicated 100 km/h | Within EU limit? |
|---|---|---|---|
| 215/55R16 | +1.6% | 101.6 km/h | Yes |
| 215/60R16 | +4.8% | 104.8 km/h | Yes |
| 225/45R17 | +0.9% | 100.9 km/h | Yes |
| 225/55R17 | +5.4% | 105.4 km/h | Yes |
| 195/55R16 | −2.4% | 97.6 km/h | No (reads low) |
Check your sizes
- 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% |
More tools
- Speedometer error calculator
- Plus sizing guide
- Tire section width guide
- Tire aspect ratio guide
- Tire size calculator
- Tire & wheel reference guides
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.
What changed
- Added curated per-size speedometer-error reference tables.