Correct speedometer gear

[mrtleavitt]

You guys can continue to duke this one out but any Ford dealership stocks many kinds or speedo gears. The manual tranny gears are different from the automatic gears. However, very many of the manual tranny gears are the same such as the NP435, T18, and T19. Those I know for sure. I can not remember exactly but I think anything from 15-19 tooth gears are available. FYI- a T19, T18, and NP435 with 3.0 diff gears and 31' tires needs a 15 tooth gear.

[fordman]

the lazy way to get this probelm taken care of. is to drive down the highway at 60 miles and hour. when you get to the first mile marker post look at the second hand on your watch. if the gear is correct then your trip to the second mile marker post will be 60 seconds. if it isnt then you adjust your speed to the next post until you figure out how far the speedometer is off. then just drive either 5 miles an hour faster or slower than the speedo reads.

[mmerlinn]

The diameter of the drive gear changes the ratio because it is a right angle worm drive not the drive teeth count. If we hit it at an angle or parallel to the Drive shaft then the teeth count would matter. But we hit it in all cars straight on the end so the diameter matters. Larger the diameter the slower it appears to spin.

Not true for speedometer gears although it IS true for ALL other gears.

With speedometer gears from ANY OEM there are usually about 3 different diameters and dozens of tooth counts. All gears of the same diameter will work with the SAME drive gear(s), but the ratios will depend on the number of teeth.

[Hawkrod]

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Not true for speedometer gears although it IS true for ALL other gears.

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That is not correct. Toothed gears of any kind work exactly the same and it does not matter what the diamters are, it is only the tooth count that matters. For example, you can have two gears with 20 teeth that are 4 inches in diameter, the driven gear will turn the same number of times as the driven gear. You can have two other gears both with 20 teeth but one is 6 inches in diameter and the other is 4 inches in diameter and the driven gear will still only turn one time for each time the drive gear turns. You can't skip teeth. Hawkrod

[SteveC]

use this formula
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.

so for me i had a manual with a 18tooth gear in it and my speedometer was reading faster than i was accually going so if it said 30 i was about 25(clocked on gps)

so NT=(18 x 30)/25 so that says i need a gear with 21.6 teeth round that to 21. I couldnt find a 21 tooth gear so i went to a 20tooth
Now using the gps methaid and the counting the mile marker, also clocked my self with one of those beside the road speed indicators. i am about 1-2 miles off on the speedometer.

though i think if you get some redicious number of teeth you need to change the gear inside the transmission or transfer case

[Hawkrod]

use this formula
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.

so for me i had a manual with a 18tooth gear in it and my speedometer was reading faster than i was accually going so if it said 30 i was about 25(clocked on gps)

so NT=(18 x 30)/25 so that says i need a gear with 21.6 teeth round that to 21. I couldnt find a 21 tooth gear so i went to a 20tooth
Now using the gps methaid and the counting the mile marker, also clocked my self with one of those beside the road speed indicators. i am about 1-2 miles off on the speedometer.

though i think if you get some redicious number of teeth you need to change the gear inside the transmission or transfer case

The sad thing is that on some trans the drive gear is actually part of the output shaft and you have to change the shaft to change the gear. Talk about a major hassle! LOL If you ever do run into that problem you can go to a speedo shop and they will fix you up with a speedometer drive adapter, it is just a little box with gears in it that can be used to correct cable speed when the trans gears can't. You most often see these on big trucks but HiPo Fords of the late 60's had them if you ordered 3.91 or 4.30 gears. hawkrod

[SteveC]

thats right i forgot that some transmissions had those gears built into the output shaft.... good info to know though thanks hawkrod

[fomocoguy]

use this formula
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.

so for me i had a manual with a 18tooth gear in it and my speedometer was reading faster than i was accually going so if it said 30 i was about 25(clocked on gps)

so NT=(18 x 30)/25 so that says i need a gear with 21.6 teeth round that to 21. I couldnt find a 21 tooth gear so i went to a 20tooth
Now using the gps methaid and the counting the mile marker, also clocked my self with one of those beside the road speed indicators. i am about 1-2 miles off on the speedometer.

though i think if you get some redicious number of teeth you need to change the gear inside the transmission or transfer case

Hey, that's very helpful! Tomorrow I will pul the gear and see what I've got. Thanks!

[SteveC]

no prob for me finding the right gear was easy.... finding the right speedo cable was a complete pain... ended up having to order one for a bronco....cause the speedo cables wernt the kind that had the gear held on with a c clip on the small stub end.

Glad i could help you