mm
%
mm
%
km/h
Result
True speed at that reading102.1 km/h
Diameter change2.10 %
How it is calculated, with your numbers
- Original diameter
d1 = r1 × 25.4 + 2 × w1 × a1 / 10018 × 25.4 + 2 × 150 × 70 / 100 = 667.2 mm - New diameter
d2 = r2 × 25.4 + 2 × w2 × a2 / 10018 × 25.4 + 2 × 140 × 80 / 100 = 681.2 mm - Diameter change
diff = (d2 / d1 − 1) × 100(681.2 / 667.2 − 1) × 100 = 2.10 % - True speed at that reading
real = shown × d2 / d1100 × 681.2 / 667.2 = 102.1 km/h
Working uses the metric units the formula is written in.
- w1 Original width (mm)
- a1 Original aspect ratio (%)
- r1 Original rim (in)
- w2 New width (mm)
- a2 New aspect ratio (%)
- r2 New rim (in)
- shown Speedometer reading (km/h)
- d1 Original diameter
- d2 New diameter
- diff Diameter change
- real True speed at that reading
What it means
A speedometer counts wheel (or gearbox) revolutions and multiplies by the circumference it was set up for. A taller tyre covers more ground per turn, so you travel faster than the reading; a smaller one makes it over-read. The error is the same percentage at every speed.
Most speedometers already read a little high from the factory, as regulations allow over-reading but not under-reading.
Assumptions and limits
- Speed taken from the rear wheel or gearbox. Front-wheel pickups are affected by the front tyre instead.
Sources
- ETRTO Standards Manual: tyre size designations (width / aspect ratio / rim diameter).