Speedometer Gear Calculations
If we already have our tires and axle, we need to find the speedometer pinion gear that correctly records distance. We can also use the speedometer gear to convert our speedometer & odometer to metric. In this example, let's assume that we have P225/75R15 rear tires on with 2.76:1 axle (47 teeth ring, 17 teeth pinion).
1957 to 1965 transmissions had 7 tooth output shafts and speedometer pinion gears ranging from 16 teeth to 21 teeth.
1966 to 1989 transmissions had 13 tooth output shafts and speedometer pinion gears ranging from 24 teeth to 45 teeth.
The 1000 turns/mile speedometer gear ratio is standard for American cars. Other vehicles (like British cars) often have other gear ratios and these are typically specified on the speedometer.
| Calculation |
US - Miles |
Metric - Kilometres |
| P225/75R15 Diameter | 28.29" | 718.5 mm |
| Corrected Tire Circumference | 7.183 ft/rev = 28.29" x 0.97 x Pi / 12 |
0.697 m/rev = 718.5 mm x 0.97 x Pi / 1000 |
| Drive Shaft Turns | 2032.1 rev/mile = 5280 / 7.183 x 2.76 |
1262.7 rev/km = 1000 / 0.697 x 2.76 |
| Required Speedometer Pinion | 26.4 teeth = 2032.1 / 1000 x 13 |
16.4 teeth = 1262.7 / 1000 * 13 |
It is possible to convert your speedometer from US miles to metric kilometers because this requires a speedometer pinion gear with less teeth. Since many American vehicles had 120 mph speedometers, the net result of this would be that the speedometer would top-out at 75 mph (ie, 120 km/h). There are 1.609344 km per mile, which is, as you can see, the ratio between US and metric pinion teeth. The only way this could work is if your tire & gear combination required a speedometer pinion gear with 37 or more teeth. (37 = 23 x 1.609344). A simpler option would be use a speedometer gear adapter that speeds up the speedometer cable by 61%.
If you're in a metric country like Canada, you're better off getting a GPS rather than changing your speedometer to read in km. Mechanical speedometers aren't very accurate but I like to have an accurate odometer for planning preventive maintenance. The GPS accurately displays your speed in km/h (or mph when you're visiting the USA) at eye-level and some have built-in trip odometers, which are handy for calibrating mechanical odometers.