The speedometer in my 79 F-150 reads about 5 mph high. I've got 3.50 gears and 235-75R15 tires on it, with a 17 tooth speedometer gear. A buddy has a truck configured identically, with a 17 tooth gear, that reads right on. I've tried all of my spare instrument clusters in it, and they all read the same, so it's not a worn out speedometer head. The cable looks to be in good shape too. Any ideas? The 17 tooth gear has to be the right one?

 

Are you sure he has the same gear? Are the tires the same circumference?

 

I guess I'm 95% sure he has 3.50's. Yes the tires are the same. Can anyone with this tire/gear combo tell me what speedo gear you have?

 

You might try an 18 tooth gear.

 

are you sure yours is reading high,maybe your friends is reading low?...

 

The output shaft in the transmission may have a different gear.

Check your speedo against 5-10 mile markers on the road or at one of those "Your speed is" setups that the police place along the road. Timing how long it takes to go one mile is not accurate enuf.

Speedometers in the USA are set up to read 1000 revolutions of the speedo cable per mile. You can tell that you have a bad speedo head if the odometer is correct but the speed is off. Always check the odometer over at least 10 miles.

 

My 78 I had to change the tranmission a year ago and the trans I put in now reads that I'm doing 15mph faster. Took it to the tranny shops and they put in a 19 gear and it still is out. The ckuster and cable and gaer are good. They say the splines that the gear matts onto in probally wore and needs to be replaced. This would require them to tear the trans tailshaft apart to replace the splines on the output shaft thatthe speedo operates from.
So I have not done this. So on the highway my speedo says I'm doing nearly 80 and I'm actually doing 60. For now I'm leaving it that way.

 

If there is any highway construction in your area, they sometimes put out the radar displays to show how fast the traffic is running. If you can find one of these, run thru it at 30, 55, 60, 70 (or speeds you will typically drive) mph displayed and see what your speedo shows. As long as you remember what the indicated speed is for the desired actual speed, you won't have a problem. This is the cheap way to resolve the problem. At 60 actual, my speedo shows 65-66. The 10 percent error here decreases as my speed drops. At 32-33 indicated I'm actually running 30.

 

I've got 235/70-15's with 3.25 rear gears and a 4-spd manual OD tranny (8 teeth on the output shaft) and I run a 19 tooth driven gear. My speedometer reads about 61 mph at a true 60 mph.

To calculate the correct number of teeth on the driven gear for your combo:
(Drive gear teeth X Ring & Pinion ratio X Tire Revs per mile) / 1000

New 235/75-15's should turn about 720 revs per mile and if you have an 8 tooth drive gear, your driven gear should be about (8 X 3.5 X 720) / 1000 = 20.2 so I'd try a 20-tooth driven gear.

I'd first try and verify the number of (drive) teeth on the tranny output shaft. Unhook the speedo cable at the cluster and attach a masking tape flag to it. Then jack up the rear wheels, put a chalk mark on the drive shaft, and make exactly 10 revolutions of the driveshaft while your assistant counts the number of revs of the flag on the speedo cable. The number of drive teeth would be the number of driven teeth X 10 (or 170) divided by the number of revs of the speedocable. (Say your speedo made 18 3/4 revs for 10 revs of the driveshaft, then the number of drive teeth would be the number of driven teeth (17) X 10 (which would be 170) divided by 18.75, or 9.066, so you would then have 9 drive teeth).

Hope this is clearer than mud

 

Thanks guys. I am positive it is off; when it reads 70 I know for a fact I'm only doing 65.

I have my transfer case tore apart right now, and from what I can tell I have a 7 tooth drive gear. Using the above formula, I come up with 17.64. My tires are far from new, so an 18 tooth would be about perfect. Thanks a lot pdqford! You did a lot of thinking for me there. I didn't realize there were different drive gears also.