speedometer gear
- Thread starter 52merc
- Start date
51 MERC-CT
Well-known member
Depends on which rear end ratio, could be anything from 18 to 22.
Motorhead6
Senior Member
As stated above, the spedometer gear choice is not dependant upon the transmission you are using, but rather the rear end gear ratio. Your drive shaft (and hence the output shaft of the transmission) will turn the same RPM for a given RE ratio at a given speed regardless of the transmission used even though the engine speed will vary. This is shown by the fact that speed is properly recorded through the range of low to high gears.
Use your GPS to determine the accuracy of the speedometer as you drive. If the speedometer reads high, slow it down by adding a tooth to the number of teeth on the gear you have. Conversely, if the speedometer reads low, reduce the number of teeth. Generally speaking, one tooth variation will change the speedometer reading by 3 to 5 MPH. Once you get to within about 1 or 2 MPH accuracy, this is as good as it's likely to get. Trial and error is the only true way to progress due to the age of the equipment you are working with and the differences in tire circuferences.
Be sure to lubricate your speedometer and cable to alleviate speedometer fluctuations.
Good luck.
rotorwrench
Well-known member
FoMoCo had this to a science back in the day. They had differences for different axle ratios and different size rear tires since both have an effect. There is a chart in the ford Chassis Parts Catalog for the 49 thru 53 models.