I have a 65 Mustang. Just bought it last week and I'm in the process of fixing a few problems. This seems like a silly problem that should be simple but it's really starting to piss me off. My speedometer isn't working. Every now and again it will jump up. I took it all apart last night but couldn't figure it out and I really don't want to buy a new one. Help!!!  

More than likely the problem is at the opposite end. Pull the speedometer cable out of the transmission and inspect the plastic gear on the end of the cable. It is most likely chewed up. This is what mainly causes needle bounce and it sounds like you are only hitting the teeth every once in a while...

Good luck...  

Hey ace_em23,

Did this fix your problem? I would like to hear about the outcome of this problem!  

I bought a new speedometer and cable. put them in last night.

So now....... Plugged everything in with the back tires jacked up and the speedometer not screwed in to the dash yet. Worked perfect. So I was all excited and let the jack down, screwed the speedo in and took it for a drive. Sure enough it wasn't working at all!!! I wanted to cry. :-) Please help!!  

Ok, now I am realllly confused. I jacked the back end up again and the speedometer works now. So it only works when the tires are off the ground!? I let it down and it doesn't do anything!! Please help before I lose my mind..  

Did you read this article?

http://www.mustangandfords.com/techarticles/75259_instrument_electricity/

"Speedometer
Unlike the rest of the instruments we’ve addressed, vintage Ford speedometers are not electrical. They’re mechanical, driven by a gear package and cable tied to the transmission’s tailshaft. Like the instruments we’ve already addressed, speedometers are simple in scope. Splines on the transmission’s tailshaft drive a small nylon drive gear tied to the speedometer cable that drives the speedometer head. The speedometer head is a magnet that spins around inside a hollow shell linked to the speedometer needle. The spinning magnet’s magnetic field attracts the shell that moves the needle. The faster the magnet spins, the higher the needle moves.

Speedometers get into trouble when cables bind or magnetic heads cease due to the absence of lubrication. Like the humble car clock, speedometers need periodic maintenance, too. The spinning speedometer head needs occasional lubrication (speedometer-head lubricant). Pull the cluster out, disconnect the cable, and feed modest doses of lubricant into the head once a year. Don’t overdo it. While you’re at it, pull the speedometer cable out and bathe it in white grease and a low-viscosity engine oil. This combination will keep it happy for thousands of miles.

Speedometer calibration is mostly a matter of using the correct drive gear at the transmission. We suggest finding a pace vehicle with a known accurate speedometer. Begin by taking the pace vehicle and heading for an open highway where there are mile markers. Get the vehicle speed to a steady 60 mph and look at your watch. If the speedometer is accurate, it will take exactly 60 seconds (one minute) to travel one mile.

If the speedometer proves accurate, saddle up your Ford and pace the test vehicle. If your speedometer reads high, you need a drive gear with more teeth. If your speedometer reads low, you need a drive gear with fewer teeth. A drive gear with 18 teeth is going to turn faster (and read faster) than a drive gear with 21 teeth. If your speedometer is reading 10 mph too high and you have an 18-tooth drive gear, you need a 20-tooth drive gear. The highest number of teeth available on a Ford speedometer drive gear is 21. After that, you must run a reducer that was common to Fords with 3.89:1, 3.91:1, 4.11:1, and 4.30:1 rearend gears. Then it’s back to the drawing board with drive-gear sizes. Be prepared to try several different sizes before lighting on the right one.

Tire size also affects speedometer readout. Taller tires will make the speedometer read lower. Shorter tires will make it read higher. Even tire inflation can affect speedometer readout.

Speedometer shops can rebuild and calibrate your speedometer. A rebuild involves new bushings and the replacement of any other worn or damaged parts. If you’ve got an unusual speedometer, such as a Shelby 0-140–mph piece, rebuilding the speedo may be your only choice. A speedometer shop like United Speedometer can help.

I am guessing that your cable is bound up when the car has all 4 wheels on the ground or that the gear in the tranny is loose/sloppy/defective when the car is on the ground and the tranny is under load.

First guess would be binding.

Let us know how it goes!

Steve  

I have ordered a new 16 tooth gear. I paid special attention when installing the new cable to make sure it wasn't bent sharply anywhere. I'll let ya know, thanks for the reply.  

Ok, new cable, new gear, new speedometer. No kinks, all lubed up. Only works jacked up. Any other thoughts?  

Just a guess, but...  You didn't say how high you were jacking the rear of the car.  Since the drive gear in the transmission is pressed on, could it be possible that when tilted forward the gear slides into place so that it makes contact with the driven gear?  The engine & trans normally tilt down in the rear.  Just a guess.  

Maybe the clip that hold the gear in the tranny is bent.  

i have a 1965 mustang that is doing kind of the same thing. the odometer works but the needle doesnt move. need help