new rear now MPH is wrong

[John Z]

Try again - step by step
1. You need to know the amount of error in your speedometer reading.
a. Indicated speed = speed your speedometer is curently reading.
b. Actual or measured speed = how fast your car is really going.
c. Measure your actual speed. You can estimate your actual speed by measuring the time it takes your car to go one mile. Most interstates have mile markers. These are small green signs on the side of the road that have a number on them. The numbers either increase or decrease depending on the direction you are going. If you measure the time in seconds it takes to go one mile, you can compute your actual speed as 3600/Time. For example, if it takes 62 seconds to go one mile, your average speed is 3600/62=58mph. If you kept your indicated speed, on the speedometer constant, e.g. 50 mph, then you can compute the error in your speedometer.
d. Error in speedometer = (indicated speed)/(measured speed) for the above, the error would be 50/58 = 0.82. This means that the cable that is driving your speedometer is only turning 0.82 revs for every rev that it should be turning. To make the cable spin faster, you need to change the gear at the end of the speedometer cable.
e. Remove the end of the speedometer cable from the transmission. For a 3 speed it should be on the driver's side. One bolt, gently pull on the speedometer cable to remove from transmission. There will be a plastic gear on the end of the cable. It is held to the cable by a "C" clip. Remove the clip and slide the gear off the end of the cable. Count the number of teeth.
f. Multiply the number of teeth by the error to determine the right number of teeth. If you have 20 teeth on the existing gear, then 20*.82 = 17.2, so you would need a gear with 17 teeth.