Thread: Ford speedo gears

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I need a gear with one more tooth and it should be right on,I have a 21 tooth gear and would like to find a 22 and anyone have any parts manuals. The parts guy at the dealer I was at the other day did not have a way to look up old part numbers,

Did you try looking on EBay? A lot of sellers there list the gears according to tooth count.

Also, it’s easy to mathematicaly calculate a gear change, so you don’t have to guess which gear to buy.

How far is your speedo off, and what speed did you check it at?

Adding one tooth to a 21 tooth gear, (1 divided by 21) will slow down the speedometer by 4.7%

60 MPH minus 4.7% equals 57 MPH. (0.047x60 equals 2.82 MPH change)

Its about 2 too fast and can not remember the speed it was checked.

A local transmission shop is often another good source for gears.

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Found none on ebay,plenty of 21 and 23 tooth gears but no 22 so I am going to leave it alone. I want to get it as close as I can to check gas mileage.

Originally Posted by junkyardjeff

Found none on ebay,plenty of 21 and 23 tooth gears but no 22 so I am going to leave it alone. I want to get it as close as I can to check gas mileage.

Assuming you are using a stock 55 Ford transmission CLICK HERE


Suggested speed to clock your speedometer would be 60mph. Believe it or not, the local police will help you do so if you ask.

Late 70s C-4

Originally Posted by junkyardjeff

Late 70s C-4

In that case, Ford, and the aftermarket do not offer a 23 tooth speedometer gear

These are your options for the C4 automatic transmission.
19 tooth-tan
20 tooth-orange
21 tooth-red
23 tooth-white

At this point, you are going to have to deal with tire size changes (small, simple, yet can be costly).

Another option is to find a measured mile (a real one, not what you see marked on the side of a typical road), take measurements, then send the speedometer itself out for re-calibration to match what you need it to be.

Looking back, you did not make mention of whether you are using an original or aftermarket speedometer. Which are you using, be as detailed as you can be, include tire sizes (rear only), rear gear ratio, type of rear end, speedometer head, any adapters, etc.

Its almost a tenth of a mile too fast according to the mile markers.

Get a GPS and it will tell you your MPH then look at your speedometer

Here is a possible solution;

Ratio Adapter

https://www.bobsspeedometer.com/1/120/ratio_adapter.asp