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Speedometer error guide

# 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.

## 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) |

## Tire size calculator

Current tire ✓ Looks valid

→

New tire ✓ Looks valid

Market EU general (±3%) Poland US (~3%) AWD / 4WD (tight)
Units
Metric mm · km/h Imperial in · mph

+ <!-- -->New tire

Requires verification Within tolerance<!-- --> · <!-- -->Diameter<!-- --> <!-- -->+1.7<!-- -->% (<!-- -->+0.43 in<!-- -->) ·<!-- --> <!-- -->US (~3%)

- **Requires verification** · This rating covers tire geometry only, not vehicle compatibility.
- Diameter +1.7 % · Ride-height change +0.21 in .
- **Speedometer reads low** · 60 mph → 61.0 mph .
- Sidewall -0.29 in · Width +0.39 in .

Diameter **+1.7 %** +0.43 in

Speedo @ 60 **+1.7 %** 60 mph<!-- --> → <!-- -->61.0 mph

Sidewall **-0.29 in** Width<!-- --> <!-- -->+0.39 in

Ride-height change **+0.21 in** Diameter<!-- --> ÷ 2

How this result is calculated

Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check vehicle placard approval, load/speed rating, rim-width range, wheel-arch, suspension or brake clearance.

[Methodology](https://tirefitlab.com/methodology/)

### Before fitting

- Check the vehicle placard and owner’s manual first.
- Match or exceed the required load index and speed rating.
- Confirm rim-width range, bolt pattern, hub bore, offset and fasteners.
- Inspect wheel-arch, suspension and brake clearance under real vehicle load.
- Check TPMS, cold pressure and local roadworthiness rules.

[Diameter **+1.7%**](https://tirefitlab.com/learn/plus-sizing/)[Speedo @ 60 **+1.7%**](https://tirefitlab.com/learn/speedometer-error/)[Width **+0.39 in**](https://tirefitlab.com/learn/section-width/)

### [Diameter](https://tirefitlab.com/learn/plus-sizing/)

**24.97 in** → **25.40 in** +0.43 in

### [Sidewall](https://tirefitlab.com/learn/aspect-ratio/)

**3.99 in** → **3.70 in** -0.29 in

### [Width](https://tirefitlab.com/learn/section-width/)

**8.86 in** → **9.25 in** +0.39 in

### [Circumference](https://tirefitlab.com/learn/plus-sizing/)

**78.45 in** → **79.80 in** +1.7%

### [Revs / mile](https://tirefitlab.com/learn/how-to-read-tire-size/)

**808** → **794**

### [Speedo @ 60](https://tirefitlab.com/learn/speedometer-error/)

**60 mph** → **61.0 mph** +1.7%

### [Ride-height change](https://tirefitlab.com/learn/plus-sizing/)

**0.00 in** → **+0.21 in**

### [True distance per 100 indicated](https://tirefitlab.com/learn/speedometer-error/)

**100 mi** → **101.7 mi** +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` · Requires verification · Within tolerance

Plan budget for this size : `235/40 R18`

Copy/print uses this page state only; TireFitLab does not create an account or store a server-side report.

[Plan budget for this size](https://tirefitlab.com/tire-budget-calculator/?size=235%2F40%20R18) Save report Print report

Embed or cite this result
HTML iframe <iframe src="https://tirefitlab.com/embed/tire-comparison/?from=225%2F45R17&to=235%2F40R18&market=us" title="225/45 R17 vs 235/40 R18 Tire size comparison" loading="lazy" width="720" height="330" style="width:100%;max-width:720px;border:0" referrerpolicy="strict-origin-when-cross-origin"></iframe> Copy Markdown link [225/45 R17 vs 235/40 R18 Tire size comparison](https://tirefitlab.com/tire-size-calculator/?from=225%2F45R17&to=235%2F40R18&market=us) Copy
[Open embed preview](https://tirefitlab.com/embed/tire-comparison/?from=225%2F45R17&to=235%2F40R18&market=us)
[Budget **$85–$120** Per tire](https://tirefitlab.com/tire-budget-calculator/?size=235%2F40%20R18&tier=budget)[Mid-range **$110–$160** Per tire](https://tirefitlab.com/tire-budget-calculator/?size=235%2F40%20R18&tier=mid)[Premium **$160–$225** Per tire](https://tirefitlab.com/tire-budget-calculator/?size=235%2F40%20R18&tier=premium)

Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.

### In-tolerance alternative sizes

| Size | Diameter | Speedo @ 100 | Use this |
| --- | --- | --- | --- |
| OE `225/45R17` | +0.0 % | +0.0 % | Use this |

<!--astro:end-->

## More tools

- [Speedometer error calculator](https://tirefitlab.com/speedometer-error-calculator/)
- [Plus sizing guide](https://tirefitlab.com/learn/plus-sizing/)
- [Tire section width guide](https://tirefitlab.com/learn/section-width/)
- [Tire aspect ratio guide](https://tirefitlab.com/learn/aspect-ratio/)
- [Tire size calculator](https://tirefitlab.com/tire-size-calculator/)
- [Tire & wheel reference guides](https://tirefitlab.com/learn/)

## Detailed reference pages

Choose a common value or size to open its calculated data and explanation.

- [205/55 R16](https://tirefitlab.com/learn/speedometer-error/205-55-r16/)
- [225/45 R17](https://tirefitlab.com/learn/speedometer-error/225-45-r17/)
- [225/40 R18](https://tirefitlab.com/learn/speedometer-error/225-40-r18/)
- [235/40 R18](https://tirefitlab.com/learn/speedometer-error/235-40-r18/)
- [245/45 R18](https://tirefitlab.com/learn/speedometer-error/245-45-r18/)
- [255/40 R19](https://tirefitlab.com/learn/speedometer-error/255-40-r19/)
- [275/40 R20](https://tirefitlab.com/learn/speedometer-error/275-40-r20/)
- [265/70 R17](https://tirefitlab.com/learn/speedometer-error/265-70-r17/)
- [31x10.50 R15](https://tirefitlab.com/learn/speedometer-error/31x10.50-r15/)
- [33x12.50 R15](https://tirefitlab.com/learn/speedometer-error/33x12.50-r15/)

[Last reviewed: 2026-06-21](https://tirefitlab.com/methodology/)

Seasonal check

## Planning a long summer drive?

Use the budget and running-cost tools before a trip, especially if the current tires are worn or the replacement size changes diameter.

[Estimate tire budget](https://tirefitlab.com/tire-budget-calculator/)

## More in this topic: Size validation

Decode markings, compare sizes, and understand diameter, load and speed changes.

- [**How to read a tire size** How to read a tire size: decode every number and letter in P205/55R16 91V — width, aspect ratio, rim diameter, load index, speed rating.](https://tirefitlab.com/learn/how-to-read-tire-size/)
- [**Tire type guide — P, LT, metric, radial & flotation** What do the letters in a tire size mean? P, LT, metric, radial (R) and flotation tire types explained.](https://tirefitlab.com/learn/tire-types/)
- [**Tire sidewall markings decoder** Complete guide to every marking on a tire sidewall: size string, construction code, load index, speed rating, DOT date, M+S, 3PMSF snowflake, UTQG, max pressure, country of manufacture, and brand-specific symbols.](https://tirefitlab.com/learn/tyre-sidewall-markings/)
- [**Tire load index chart** Use the tire load index chart to convert ratings 60–130 into kg and lb, then verify the vehicle placard and axle-load requirements.](https://tirefitlab.com/learn/load-index/)

### Useful tools

[Speedometer error calculator](https://tirefitlab.com/speedometer-error-calculator/)[Compare tire sizes](https://tirefitlab.com/compare/)