Rifleshooter
Well-Known Member
So one of the many challenges I'm facing with the restoration of my 80 volare duster is that my speedometer is around 10 mph too slow.
Correct.the gears from the plastic housing and metal housings do not interchange.
I understand everything youre talking about Bud, my only question however is how to i end up finding what rear gear i have without having to take off the diff cover. My diff currently has tiny metal shavings in it and id prefer not having to change it out just yet as it is one of the later things on my list of things to fix or replace.Slant6billy is on the right track – but before you change speedometer gears (in transmission), there is two different items in play. Generally the two items are tied to each other and other times not – but you need to check first.
Your Odometer reading needs to be the first thing to correct, then retest to see of speedometer is off, or not.
What I would do is find a section of highway and drive at least 10 miles. More is better – but 10 miles is easy to divide by.
Either record your odometer reading (or take a cell phone picture) right at the mile marker.
Drive 10 miles and right at the mile marker, again, record your odometer reading.
This way, the tire size and differential gear ratio won’t be to much of a factor.
My example, I drove my ’77 wagon from Oklahoma City to Waco at the first of the year.
Along the way, I recorded:
39,642.3 At beginning of test
39,695.2 After traveling 50 miles.
53.9 miles accumulated in a 50 mile trip.
53.9 Miles times 2 = 107.8 miles traveled in 100 actual miles – or 7.8% difference.
My wagon came originally with F78-14 Firestone Super Belt Deluxe Champion tires (Wanted - 77 Volare rear leaf springs post #53 ).
Those tires are 26.5” tall and go on a 5.5” wheel.
The tires currently on car are not the correct size.
This car came with 2.9 gear ratio (either a 2.93 or 2.94) so speedometer gear should be a 29 according to the chart above.
According to the parts manual, F78-14 with 2.93 has a 29 tooth speedometer gear (black).
If a 2.94 ratio, then 30 tooth speedometer gear (yellow).
I know I have 2.9 but not sure which 2.9 (2.93 or 2.94) so it could be with either speedo gear.
Sense I’m recording more miles than actually going, I need to slow it down 7.8%
29 teeth - 7.8% = 26.74 (rounded to 27 (white))
30 teeth - 7.8% = 27.66 (rounded to 28 (blue))
So to get my odometer reading correctly, I need to have on hand a 27 (white) or 28 (blue) – depending on speedo gear I currently have.
To know what speedo gear I have, means I need to get under car. If you have rear of vehicle up high enough, you will not lose much ATF. Remove the speedometer clamp (7/16" bolt) then gently rock the cable assembly back and forth and unit will pop out of transmission.
The gear will be stamped what number it will be as well as the gears are colored plastic (red, orange, white, blue, etc.) - so if you can't read the number, at least get color.
With tire size, we can determine which gear you have.
One that is done, I need to retest to make sure odometer is reading correct.
Now I am able to test the speedometer reading (which I also know is not correct on my wagon).
- - - - - -
A speedometer uses a U shape piece of metal and a rotating magnet. Watching a video on internet might be easier than explaining how it works – but the way to adjust the speedometer is to lightly bend the tangs of the U-shaped piece of metal that goes around the rotating magnet inward to speed it up (or outward to slow it down). I do mean lightly bend, for thousandths of an inch makes a difference (0.001”).
Also, anytime you have a cable driven speedometer head removed, you also need to add light oil to a few key parts and to the cable, so speedometer needle doesn’t jump around.
I know where at on the ‘60’s speedometers but need to look at one of ours to tell you where at, on ours.
Calibrating a speedometer head (once odometer is correct) might be best left to a speedometer shop (if any are left out there) – for the area to work in is rather cramp.
To summarize, there are two calibrations to speedometer assembly.
Calibrate (or confirm) Odometer first, then go on to speedometer calibration.
How to test Speedometer calibration – is a different topic.
BudW
Well going off of the chart toward the top of thus page I'd say I'll most likely need a 28 tooth gear, but before I give a define answer I'll need to find out what rear gear I have for sureI have a couple NOS gears. What tooth count did you need?
So if one had the metal housing from any other (and older) 904 trans, they could swap it in for a plastic one?The biggest trouble when using the plastic housing is actually finding a speedometer drive gear. They are not easy to come by and to find one with the correct number of teeth can sometimes be like,,,,,well,,,,,,,pulling teeth.
If you are unaware, the gears from the plastic housing and metal housings do not interchange.
Side note: My take on why they went to plastic is,,,,,,it's most likely cheaper. Isn't that always the way?