I just bought a 140 mph speedometer for my 85. It reads about 5 mph slow at all speeds though. It doesn't come off the stop until i'm doing probably 15. It was supposed to be calibrated. I still have the stock 2.73 gears and speedo gear. Any suggestions?

 

might be that it was calibrated for 3.08 gears, is it the ford motorsports or SSP version?

 

that is a good point..but i'd make sure connections are tight...my 3.73 gear went crazy and chewed it up cause i didn't push it all the way in.

 

have to change the gear on the speedo cable

 

Change it to what? I need to get the cable to turn faster. So put an 18 tooth in?

 

first you need to see what gears you got..stock?? or added a set?? if not sure and rear end tag is missing..you can remove the cover ..count teeth on ring and pinion and divide numbers and there you have it the ratio

 

i know i have 2.73's. To get the cable to spin a little faster. Should i get a speedo gear wit hmore teeth or fewer? Going up would be to an 18 tooth i assume.

 

Hey, I love speedo questions! Background -I used to manage a Speedometer shop here in KC - Kansas City Speedometer. They are out of business now. Sorry this is a bit long...

Any who, US automanufacturer mechanical speedometers, regardless of the max speeds they register on the dial are all calibrated to the same spec - 60 mph (+ or- 2%) is shown at 1000 cable rpm. So, when replacing a speedometer in a car - even one that registers a higher max speed - it is not necessary to change anything on the car. The new speedometer will be just as accurate as the one it replaces.

Now it is possible to get a new speedometer that is not calibrated correctly. Mechanical speedometers use magnetisim to move the indicator needle. If a speedometer come in contact with an electro-magnet field or pulse it can lose calibration and be inaccurate.

How to test: Use the odometer because it is gear driven and not affect by instrument calibration.

Note:This applies to any situation - IE new speedometer, rear end gear ratio change, and or rear tire size (diameter) changes.

Install the speedometer and make sure that the cable is securly snapped into place. Drive the car on a highway that has mile markers and compare the mileage registerd by the odometer to the mile posts that you are passing. Testing accuracy is better if you do this test over the course of 5 - 10 miles. A properly calibrated speedometer - meaning that the car has the correct speedometer drive and driven gears in the trans will record 5 miles for 5 miles of actual driving.In other words if you drive 1 actual mile the odometer should record 1 mile exactly. The amount of variance can be calculated to determine the percentage of error.

For Example:

Overa 5 mile distance your car records 5.5 miles on it's odometer. The amount of error is 10% and in this case the cable is spinning 10%faster than it should.This could be causes by a 10% reduction in rear tire diameter or a 10% increase in final drive ratio. To fix this you would need to reduce cable speed by 10% by installing a driven gear (the one on the end of the cable) with 10% more teeth than the one installed during the test. So if you had a 17 tooth gear go to an 19 tooth gear.

Okay, back to the new speedometer. If you install it and verify that it records mileage correctly then you know that you need to make no changes to your car. If you still think it is not accurate with respect to speed you can use a stop watch and mile posts to check accuracy. Simply,your car should cover 1 mile in 1 minute at 60mph. If the new speedometer fails your speed tests, it is defective (again, assuming that it records mileage correctly)

As I recall, high MPH speedometers are a little sluggish at low speeds because their scale is a bit more compressed than a standard 85mph speedo. Comparing the two instruments I think you will notice that 60MPH is closer to the 0 mph peg on the 140 speedo than it is on the 85 mph speedo.

Hope this helps

 

+1, if it was for the ssp its gonna be for 3.08 gears

 

I agree with Mustanghammer on this one or Mongolchuck is incorrect on the original diagnoisis.

Speedometers error of 5mph at ALL speeds is NOT the fault of the rear gears or trans speedometer gear.

Reason is any gear error will multiply the faster you go.
A 5 mph error at 50 will be 10 mph error at 100. That is not what the original poster describes.

The original complaint is 5mph at all speeds. Seems like reclocking the needle over 5mph will cure his needs.

As for 1 mile actual vs 1 mile on the odometer. BEST ANSWER.