When you buy a vehicle, new or used, the calibrations have been set, at the factory, to the specifications listed in the owner’s manual for that vehicle. During the life of the vehicle, changes are made that alter the original calibrations. One calibration that changes is the speedometer.
Speedometers are calibrated to the rotation of the transmission, or in older vehicles, the rotation of the drive shaft. The speedometer will lie to you if the tire pressure has been changed from the specifications in the owner’s manual. The tire and/or rim size is also a factor to the speed you are actually traveling. If your vehicle came with 14-inch rims and you replace them with 15-inch, your vehicle will be traveling faster than the speedometer indicates, because the wheel travels a longer distance in each revolution. A deeper sidewall tire will have the same effect. Although your speedometer is telling the truth according to the way it’s designed, it is giving a false reading to your actual speed.
One way to test your speedometer is to travel a highway that has mile markers. Get your speed up to 60-miles-per-hour and time your travel from one marker to the next. It should take one minute to travel one mile. If it takes less than a minute, you’re going faster than indicated on the speedometer.
The odometer is the meter that tells you how many miles you have traveled. Newer vehicles have a digital readout, while older vehicles have a rotary system. Some digital speedometers don’t show the tenth of a mile. Usually the trip set readout will give you the tenth reading. Check your odometer accuracy the same way as you did with the speedometer. It doesn’t matter how fast you drive between the milepost numbers because the distance is the same. Your odometer should show exactly one mile, to the tenth, between the mileposts. A faulty speedometer or odometer will also give you a false reading on your gas mileage.
Speedometers and odometers are both affected when changes are made. The required air pressure in the tires and the size of tires or wheels are part of the calibration. To correct the problem, refer to the owner’s manual and go back to the original specifications for that vehicle. There is a small possibility that the settings were in error when the car left the factory, but putting the original equipment back to specs will bring you closer than trying to reset the readouts. Original settings for tires and wheels will also improve your gas mileage, because a larger tire causes more drag on the road and a larger wheel needs more horsepower to turn it.