here is another:
http://www.transmissioncenter.net/speedometer_calibration_______va.htm Besdes having all the necessary gears for just about anything they also have ratio reducers and ratio increasers to cover what cannot be had with a simple driven gear change.
One of the easiest ways I've found to figure out the error all by youself is to find a length of road with Measured Mile signs. Note the 1/10 odometer number at the start of the measured mile and again at the end. Simple enough to figure out if your odometer is turning faster or slower and by how much. Like if it reads 1.2 miles then it is 20% too fast as an example. Adjust your cable driven ratio by that much. The beauty of this is it works at any speed. A mile is a mile whether you drive it at 20 or 60.
Interstate highway mileposts are another good place to check your speedo. Most are posted every 1/10 or 2/10 of a mile. Driving 10 miles on the interstate for the error check will really give you an accurate reading.
