Speedometer Correction Table Generator
Make a table of shown speed and true speed. Use the calculator below. Results update as you type.
How the Speedometer Correction Table Generator Works
This generator works by applying the tire-size factor to a list of indicated speeds and printing the true speed next to each one. It produces a ready-to-print reference card.
- Type the factory size, for example 265/70R17.
- Enter the new size, for example 315/70R17.
- Choose the speed range and step, for example 10 mph to 80 mph in 10 mph steps.
- Read or print the table of indicated vs actual speeds.
The Formula
The formula applies Actual = Indicated × Factor to every row, where Factor = New Diameter ÷ Old Diameter.
Factor = D_new ÷ D_old = 34.36 ÷ 31.61 = 1.0872 Actual_i = Indicated_i × Factor Gap_i = Actual_i − Indicated_iWorked example row: indicated 40 mph × 1.0872 = 43.5 mph, a gap of +3.5 mph.
Conversion Table
The conversion table is a generated example for a 265/70R17 to 315/70R17 change, with 8 rows from 10 mph to 80 mph. Each row lists the gap in mph and the matching km/h values.
At 80 mph indicated, the truck runs at 87.0 mph. That 7 mph gap can turn a legal cruise into a speeding offense.
| Indicated mph | Actual mph | Gap (mph) | Indicated km/h | Actual km/h |
|---|---|---|---|---|
| 10 | 10.9 | +0.9 | 16 | 17.5 |
| 20 | 21.7 | +1.7 | 32 | 35.0 |
| 30 | 32.6 | +2.6 | 48 | 52.5 |
| 40 | 43.5 | +3.5 | 64 | 70.0 |
| 50 | 54.4 | +4.4 | 80 | 87.5 |
| 60 | 65.2 | +5.2 | 97 | 105.0 |
| 70 | 76.1 | +6.1 | 113 | 122.5 |
| 80 | 87.0 | +7.0 | 129 | 140.0 |
What Is the Speedometer Correction Table Generator?
The Speedometer Correction Table Generator is a tool that builds a custom indicated-vs-actual speed table for your exact tire change. The output is a reference card you can keep in the vehicle.
The generator saves you from mental math at speed. It covers both mph and km/h, so it works for drivers crossing between the US and Canada or the UK and Europe.
Common Uses
This generator has 4 uses: dash cards, fleet handbooks, cross-border driving and inspection records.
- Print dash cards: Keep a quick lookup on the sun visor.
- Write fleet handbooks: Give drivers a table for each vehicle.
- Drive across borders: Read mph and km/h on one card.
- Record inspections: Attach the table to modification records.