Speedo gear.

[luke-ao]

Not sure which forum this goes in...

My speedometer reads 50 mph when my actual speed is 70 mph, do I need more or less teeth on my gear?
is it 10 mph per tooth?

[chad67]

There is a formula to find out how many teeth you need on your speedometer gear and I have it somewhere. I am not at home where my library is, but as soon as I find it, I'll post it for you.

[Art]

You need a gear with fewer teeth on the speedometer cable (the driven gear). Another way to fix it would be a drive gear (on the transmission output shaft, or transfer case output shaft if 4wd) with more starts (i.e. "teeth").

If you are travelling 70 mph and the speedometer reads 50, you need to change the speedometer gear ratio by 50/70 or 0.714 Another way to look at it is to see if there is a speedo cable gear with 5/7 the number of teeth. Or a trans drive gear with 7/5 the number of starts (1.4 times as many).

What caused this issue - changing gear ratio or tire size or both? If this issue developed without parts being changed, perhaps the speedometer itself should be looked into.

I had to correct the speedometer on my 69 F100 when I swapped rear axle ratio from 3.50 to 2.75 and it is dead-nuts-on now. Even been confirmed by radar...

[chad67]

The formula is:


(drive gear teeth X rear end ratio X tire revolutions per mile) divided by 1000=the number of speedometer gear teeth you need

[Art]

The formula is:


(drive gear teeth X rear end ratio X tire revolutions per mile) divided by 1000=the number of speedometer gear teeth you need

Makes sense, as these speedometers are designed to read 60 mph (one mile a minute) at 1000 speedometer cable RPM.

[chad67]

I found this formula in an old Ford SVO catalog while I was trying to figure out which gear to use on my T-bird with 1 inch taller tires going from 2.73 gears to 3.73 gears. It works.

[luke-ao]

I did some searching and one guy suggests that each tooth is 3.375 mph (+-) per tooth, no clue if it works. My gear has 19 teeth and the lowest number I can find on a ford gear is 16.

What caused this issue - changing gear ratio or tire size or both? If this issue developed without parts being changed, perhaps the speedometer itself should be looked into.

I've had the truck about 20 years, it was like that when I got it.

[chad67]

What tranny are you running, what rearend ratio, and what size tires?

[chad67]

And BTW, you can not use the # of teeth = XX MPH conversion as speedo error is a percentage. An error of 1 MPH at 10 MPH will be an error of 7 MPH at 70 MPH (10%).

[ultraranger]

Ford drive gears (the gear on the transmission output shaft) comes in 6, 7 & 8 tooth counts. The driven gear (the gear on the end of the speedometer cable) comes in 16-21 tooth counts. There are aftermarket 23-tooth driven gears, for low geared (numerically high) rear ends but, the teeth on them are fairly thin and they tend to not last long.

The majority of Ford transmissions (manual & automatic) have speedometer cables that enter the tail housing on the left side (this would be a Ford Type-3 driven gear). Ford Top-Loader 4-speeds have cables that enter on the right side. This means you would have to have a driven gear with teeth that run in the opposite direction of the teeth that would be used on a cable that enters the left side of the transmission. (the Top-Loader 4-spd. uses a Type-3A driven gear).

If the calculation in number of driven gear teeth is something other than a whole number like; 18.6, for example, then you would round up to the nearest tooth count of 19. If it comes out to 18.3, (again, just for example), then you would round down to the nearest tooth count of 18.

[Art]

And BTW, you can not use the # of teeth = XX MPH conversion as speedo error is a percentage. An error of 1 MPH at 10 MPH will be an error of 7 MPH at 70 MPH (10%).

Right on

Too many people make that mistake...

[luke-ao]

The tranny is a C6
3.25 rear
tires are 27" tall

[luke-ao]

I found this cool little calculator online>
http://www.tciauto.com/tc/speedometer-gear-calculator/

The C6 that I just installed is one I purchased to go with the fresh 302 for the 390-302 swap I'm in the middle of. I just climbed under the truck to see if there was anyway to count the teeth with the trans installed and noticed that the hole was plugged when I pulled the plug and stuck my finger in the hole, all I found was a smooth surface, no gear. Checked the old C6 I just pulled and could definitely feel a spiral gear. Is it possible my slip yoke is too long and is covering it?

[chad67]

No, the slip yoke can't slide far enough to cover the gear. I would definitely check to see why, though. I know the output shaft on C6's can be different lengths and splines. Your shaft and extension housings may not be compatible. Better to find out now. Also count your speedo drive gear teeth so you can find out which driven gear to buy.

[luke-ao]

What do you mean not compatible, it fits. (?)