I recently installed a speedo cable on my truck, it hasn't had one since I bought it. Anyway, now the speedo registerss but WAY too high. It's wrong across the whole range, when my gps indicates 20 mph, my speedo indicates 45. When I'm getting 65 on the GPS, I'm getting over 100 as a reading on the speedometer.

1979 D100, automatic, with 235/70-15 tires.

Do I have the wrong cable or something??

 

the tires are a bit smaller but not enough throw it off by 25mph. it should only throw it off by about 2mph if you where going 60mph so you would only be doing 58mph. what is the max speed on your speedo? maybe it has been replaced and they used the wrong one.

 

It's a 100mph max speedometer. If I'm going 65, it pegs the speedometer needle.

 

Since this has never worked since you have owned it, four things come to mind.
1. Is the speedometer original equipment?
2. Is the transmission original equipment?
3. What speedometer pinion gear is inside the transmission that drives the speedometer cable?
4. Some applications require a reduction gear box and some don't. If required this gear box fits between the speedometer adapter that bolts into the transmission and the speedometer cable. My truck uses a reduction gear box, it is stock.

 

could also be that someone changed the gears in the rear end or swapped rear ends with another truck with different gears.

 

return cable and continue using gps problem solved your welcome

 

SEAL and crazzywolfie, thanks for your helpful posts. jram, not so much.

The trucks history is mostly unknown to me. May or may not be the original transmission, I know it's not the original engine. Speedo head may be original or not, don't know. Also don't know what gears I have in the rear end. All unknowns.

I may take it to a speedometer shop after I sort a few other things out, and see if they can get it working correctly.

 

Look on the drivers side of the transmission on the flange that the pan bolts to. You will find 3 sets of numbers. Post these numbers here and I will try to help you further and possibly save you some money.

 

I will do that tonight when I get home from work. Thanks!!!

 

Okay, I've got those transmission ID numbers.

PK4202571 7536 9986

The shape of the pan looks like this:



Looks like I've got an A727.

While poking around under there, I "think" I've found part of my missing neutral safety switch wiring. I have the NSS in place on the transmission, but the wiring pigtail is missing. Over on the drivers side frame, I found the stub ends of a black wire, and a red/black stripe wire. I think theses are two of the connections I need to get my NSS working again. No sign of the third wire, which, IIRC, is supposed to be a green wire.