Classic/Antique Car Repair: 1964 Ford Pickup, differential gears, speedometer cable

Question
Hi Mr. Benjamin,
My vehicle doesn't seem to quite fit your described expertise, but thought you might be able to point me in the right direction -- if there is one.

I recently had the differential gears changed from 3:89 to 3:00 in my 1964, half-ton, short-bed. 292 Y-block pickup (which I bought new). The higher-geared differential has been great on the freeway, but the residual problem is the speedometer and odometer. Both are under-recording speed and miles respectively. I called one shop in my area that deals with this issue, but they said my vehicle is too old for them to fix. Is that true?

Can an adjustment be made? Doesn't the speedometer cable go into the transmission?. Does the cable have a gear in the transmission? Should or can the cable gear be changed? If so, where do you get the part and how do you figure out what is needed?

I should note that several years ago I swapped the stock three-speed for a Mustang 5-speed stick. Afterwards, there was no change in the speedometer reading because either the mechanic made an ajustment at the time or the swap had no affect on the speedometer.

It seems that the I'm underrecording the speed by about 8 miles an hour at 40 miles and hour. I've heard that the error factor increases with speed. But others say that isn't true.

Thanks in advance for any advice or help you can provide.

Bill Packer

Answer
Your question is a generic question, it does not depend on the vehicle involved (although, as a matter of fact, I happen to have a 1964 Ford Pickup, although mine is an ex-army 4X4 and a 3/4 ton - and mine is also geared way too  high for use on the highway, but since I use it mostly on my ranch, it really doesn't matter to me).

Anyway, back to your question:  The speedometer is gear driven by a set of gears in the rear housing of the transmission - so what it is really indicating is the revolutions of the driveshaft.  Any error caused by installing a different rear end ratio is strictly proportional to the change in gear ratio of the rear end.  Since you put in a 3.00 instead of the original 3.89 gear ratio, your speedometer (and, of course your odometer too) will underreport your speed and distance by that exact ratio, namely reporting 3/3.89 or 77% of the true speed and distance.  This is of course a constant error, and will be that percentage regardless of speed.  When you changed transmissions, you didn't change the rotation of the driveshaft because the gear ratio of the rear end was not changed, thus there was no correction required.

In your current case, you will have to do some research to find out if there was any other speedometer drive gear set that can be installed in your current transmission that will spin the speedometer drive cable 1/.77 or 30% faster - this will bring your indications of speed and distance back to what they were before the rear end change.

If you determine that there was an optional rear end ratio for a Mustang that was close to 3.00, you can buy a set of speedometer drive gears and install them in your Mustang transmission - however this will take a professional mechanic to do because the rear housing has to be partly disassembled to make this change.  There are two gears in the set, one is easy to change (the one on the end of the cable), but there is a mating gear inside the transmission that also has to be changed so that the gears will mesh properly.

If you do this, your readings will still be somewhat in error because the tire sizes are probably larger on your truck than on the Mustang.  If you adjust the gear ratio to compensate for that, you can get your speedometer and odometer right back to accurate.

By the way, the person who told you your vehicle is "too old" to work on is wrong, the change has to be made to the Mustang transmission, which is surely much newer than your truck.   If you take your truck to a speedometer shop, I'm betting they can fix you up easily with a recommendation for the right parts, then take the parts to a transmission shop and have them installed.  They can also remove and re-calibrate your speedometer to read right with the current gears - I'm not sure which would be a better way to go - ask the Speedometer guys.

If this is all too much to contemplate, you can always re-mark the speedometer face with a new set of numbers, marking "40" where the old "31" was and so forth.  In the mean time, mentally compute .77 times the speed you want to drive and make the speedometer read that number, you'll be pretty close.

Dick