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Speed Limit Compliance Calculator

Work out speed limit compliance instantly with clear inputs, formula shown and shareable results.

About Speed Limit Compliance Calculator

Speedometers are legally required to never under-read, so they typically over-read by 2-5%: an indicated 78 km/h may be 76 actual. Enforcement tolerances are separate and discretionary, commonly 10% plus 2 km/h, and are never a legal entitlement.

Formula

Speed check

actual = indicated / (1 + speedometer error); threshold = limit x (1 + tolerance) + 2

Worked example

With the default values below, Speed Limit Compliance Calculator returns Actual road speed of 75.73. Change any input above and every figure here is recalculated in your browser.

Inputs used

Speed limit
70 km/h
Your speed
78 km/h
Speedometer over-read
3 %
Enforcement tolerance
10 %

Results

Actual road speed
75.73 km/h
Amount over the limit
5.73 km/h
Typical enforcement threshold
79.00 km/h
Status
Over the limit but within typical tolerance

Step by step

  1. 1Values used
  2. 2Speed check
  3. 3Actual road speed= 75.73 km/h
  4. 4Amount over the limit= 5.73 km/h
  5. 5Typical enforcement threshold= 79.00 km/h
  6. 6Status= Over the limit but within typical tolerance

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why do speedometers over-read?

Regulation requires that they never show less than actual speed. Manufacturers build in a margin so tyre wear and pressure variation cannot push them under.

Can I rely on the enforcement tolerance?

No. It is guidance to enforcement agencies, not a legal allowance. The limit is the limit.

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