Advance Design truck speedo calibration

[50jimmy]

Hello Gents.  I have finally gotten the jimmy out on the road. However the speedo has quit working. It worked for a while, and was registering about 45 mph at 65 mph or so.

Some of the online speedo gear calculators want to know how many revolutions the speedo needs to register a mile. Would anyone happen to know this?  Or could you direct me to where I could find such information?


Thanks a bunch...Scott

[enjenjo]

A properly set up speedometer will have the cable turn 1000 revolutions per mile. There are a couple ways to check this. I have a speedometer calibration kit that you fasten to the cable in place of the speedometer. You set it to 0, and drive the car for a measured 10th of a mile, 528 ft. The counter on the calibrator should read 100 over that distance. The amount it is off is the pecentage faster or slower it has to turn to be correct. You can use this with the available gears to correct it.

The other way takes nothing more than a working Odometer, correct or not. using mile markers on a highway, set a trip odometer to 0, or note the reading on a non resetable odometer. Drive exactly 10 miles. The number of 10ths of a mile you are off is the correction percentage.

If the gears available are not enough to correct it, you can get a ratio change box off an old 70s chevy truck, or most old class 8 trucks http://www.gaugeguys.com/speedcorrect.htm You can buy gears for these to change the ratio to about anything.

[50jimmy]

I think my question was to be more exact......I understand that a thousand turns of the speedo cable per mile is pretty standard after 1965 ( or so says the internet), I am wondering if my 49/50 speedo in my gmc would be designed to this standard. (the gear head in the dash unit)

I havent isolated the cause of the speedo's demise yet for sure. I disconnected the cable at the back of the instrument and the cable  would turn freely. I disconnected it at the transmission, and put a drill on the cable and spun it, and the speedo registered movement more or less in synch with the variation in speed of the drill.  I am operating on the assumption the gears in the tail housing are defunct in some way, although I haven't had time to pull the tail housing and have a look. I'm hoping to come pretty close to accurate by swapping out the plastic gears in the tail housing.

[enjenjo]

Yes the GMC speedo is 1000 revs per mile.

The gear on the tailshaft seldom goes bad, it's usually the driven gear on the speedo that goes bad. I have also seen the cable drive socket in the driven gear worn bad enough that it won't drive the cable. There are charts online that will tell you what gears are available for your trans. The drive gear on the tailshaft does not change, it fits all driven gears.

The speedo itself may just need a good cleaning, with air, and lubed with 3 in one oil. Don't use too much. It has jeweled bearings like a watch.

[50jimmy]

I think the speedo itself is ok, I had it apart and cleaned it  up and oiled it  when I redid the face and odometer.

Thanks for the information and advise, enjenjo.  It'll be a day or two before I can get it up on jacks and take a look at the driven gear and see if the drive gear is ok and which one it is.  Might be the speedo cable is "skinny" also and complicating things since it came from one of the reproduction places.

I drove it to work monday.  It still doesn't look like much and it won't ever be a show truck, but it is what I envisioned with the skills, tools and money I had to do  it.   Its a tire squealing lil buggy, even with the 2.41 rear end.