TireFitLab

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.

These rows use the same geometry engine as the calculator, so the numbers match the interactive result above.
Current sizeCandidate sizeDiameter deltaSpeedometer at 60 mphTolerance
225/45R17235/40R18+1.7% (+10.9 mm)60 mph → 61.0 mphwithin
205/55R16225/45R17+0.4% (+2.4 mm)60 mph → 60.2 mphwithin
215/65R16225/60R17+2.3% (+15.9 mm)60 mph → 61.4 mphwithin

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

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Units
Requires verificationWithin 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 inWidth +0.39 in
Ride-height change+0.21 inDiameter ÷ 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

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
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%
Ride-height change
0.00 in +0.21 in
True distance per 100 indicated
100 mi 101.7 mi+1.7%
Δ⌀ +0.43 inΔ↔ +0.39 in
  • 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

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Plan budget for this size
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Budget$85–$120Per tireMid-range$110–$160Per tirePremium$160–$225Per tire

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

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
OE 225/45R17+0.0%+0.0%

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Detailed reference pages

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Last reviewed: 2026-06-21

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