How to use the speedometer error calculator
- Choose whether you know the true speed (from GPS, for example) or the tire diameters, and pick mph or km/h.
- Enter the speed your speedometer shows.
- For the true speed option, enter the true speed. For tire diameters, enter the original and new overall diameters in the same unit.
- Read the error, the true speed, the correction factor and the table of other readings.
Formula
Correction factor = true speed / shown speed
Error = (shown speed - true speed) / true speed x 100%
From tire sizes: true speed = shown speed x new diameter / original diameter
True distance per 100 on the odometer = 100 x correction factor
A positive error means the speedometer shows more than the true speed. A negative error means it shows less.
A speedometer reading 65 at a true 62
If the speedometer shows 65 mph while GPS says 62 mph, the correction factor is 62 / 65 = 0.9538 and the error is (65 - 62) / 62 = +4.84%. The speedometer reads high. A reading of 60 means a true speed of about 57.2 mph, and 100 odometer miles are about 95.38 true miles.
For tire sizes, a change from a 28-inch to a 29.4-inch tire is 5% larger. If the speedometer was set up for the 28-inch tire and shows 60 mph, you are really going 60 x 29.4 / 28 = 63 mph, an error of -4.76%, so it reads low.
Why larger tires change the reading
A speedometer counts wheel or driveshaft turns and assumes each turn covers a set distance. A taller tire covers more ground per turn, so you travel faster than the dial says, and the odometer also records fewer miles than you drive.
The relationship is proportional to overall tire diameter, which is why a tire 5% taller makes a 5% difference in true speed. Gear changes at the axle have the same effect through the ratio.
Measurement limits
GPS speed is itself an estimate and usually accurate within a small margin, and tire wear and pressure change rolling diameter. Check against a stable highway speed and repeat to be sure.
If you know only a tire size code such as 225/65R17, the tire size calculator works out the diameters for you. The gear ratio calculator shows engine speed for a given tire, and the MPG calculator shows how a wrong odometer affects fuel economy.
Assumptions and limits
- The true speed or the tire diameters you enter are correct.
- Error is relative to the true speed, not to the speedometer reading.
- Both tire diameters use the same unit.
- Speed is steady and not affected by wheel slip.
Frequently asked questions
How do you calculate speedometer error?
Subtract the true speed from the speedometer reading, divide by the true speed and multiply by 100. A positive result means the speedometer reads high.
Why does my speedometer read low with bigger tires?
A bigger tire covers more distance per turn, so you travel faster than the speedometer shows if it was set for the smaller tire. The same applies to the odometer.
Is GPS speed accurate enough?
It is usually close at a steady speed, though it can be off by a small amount. Compare at several speeds, and treat a single reading with some caution.
Is a speedometer error a problem?
This tool only measures the difference you enter, and says nothing about what error is acceptable. Tire wear, pressure and size changes all move the reading.