Quote:
Originally Posted by Piwoslaw
I have to set the cruise control to 73 km/h for the GPS to show 70 km/h (more than 4% off), but at 100 km/h the cc also has to be 3 km/h more (103 km/h, only 3% off). It appears that the difference depends on speed?
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Actually, it's not unusual for a speedometer to be +/- 5%, especially if it's a mechanical "eddy current" type. For FE, the question is, is the odometer accurate? If you have a mechanical speedometer/odometer, there is a direct mechanical relationship between the number of times the transmission output shaft rotates and the number of times the odometer digit is advanced. Using a mileage check with highway mile markers will tell you how close your's is, but this will always be "off" a little, based on the fact that there are only so many combinations of teeth in the possible gear sets available, and the fact that tires are never "exactly" 25.3" in diameter (or whatever the nominal diameter is for the tire on your car.)
The latest speedometers are digital, and "count" the number of pulses from the car's VSS (electronic sensor, usually attached to the transmission's output shaft as well, but on some of the new cars, counting pulses off of a front brake's anti-lock sensor) that are calculated to make up a mile. Obviously, you can get a lot closer with these, since even if the VSS sends only one pulse per output revolution, if you have a 3.9 final drive and 25.3" diameter tires, and the speedometer has to round off to the nearest pulse, it could only be something like 10" off every mile at the most. Of course, then there's tire wear.
But most aftermarket electronic speedometers allow you to calibrate the speedometer as often as you like.
But if you DO have a mechanical speedometer, just check the accuracy of the odometer against a verified distance (interstate mile markers are handy, but measuring 10 miles is 100 times better than one), calculate the odometer error, then correct your logged mileage numbers accordingly.