My 1980 W150's speedo gear has all it's teeth smoothed off(I think from the retainer bolt backing out and the retainer slid out). I'm hoping it's not from the cable binding up...

Here's my question: I'm going to get a set of new tires soon and I'm not sure which tooth count I need to go with 33-35" tires? I'm not sure of the rear ratio because the tag is gone. It has the Dana 44 front and Chrysler 9 1/4 rear. The old speedo gear was 40 tooth and it was approx 25mph off with 35" tires.

Thanx for any help!

You need to know the gear ratio and the tire diameter. Here is a post on how to find the gear ratio. https://board.moparts.org/ubbthreads/ubbt...gear-ratio.html

Here is a chart you can use to figure out the correct gear. Just see what an inch of tire diameter makes in gear teeth with your gear ratio and go from there.
http://www.moparts.org/Tech/tech/pages/speedo.html

Thank you sir!

take a quick look at Dragtimes dot com spreadsheet----it is very easy--and lets you put in all the gear ratios/diff ratio/and tire sizes....then it shows the rpm increments...pretty simple setup...

for figuring how many teeth you need on the gear--- look at go.jeep speedo gears---

On a mechanical speedometer (especially an older one) I usually figure it this way.

1. Pull the speedometer gear and get a tooth count. If you don’t have a gear, put a speedometer gear in it (it really doesn’t matter how many teeth) but you do need to know the number of teeth.

2. Take it for a test drive (ideally at highway speeds) with either a GPS or a follow car with an accurate speedometer.

3. Once you have established what the real vehicle speed is compared to what the speedometer reads divide the small number by the larger number:

i.e. Vehicle is travelling at 65 MPH speedometer reads 72 MPH

65 divided by 72 = .902 This is the percentage the speedometer is off.

If the speedometer reads slower than the vehicle is actually travelling you need a speedometer gear with less teeth to speed up the speedometer and if it reads faster than you are going you need a gear with more teeth to slow it down.

Say the speedometer gear in the transmission has 33 teeth….you will need a gear with less teeth……..start plugging numbers in the calculator again dividing the smaller number by the larger until you end up with a percentage closest to the percentage the speedometer is of .

In this case a gear with 30 teeth divided by the 33 teeth (of the gear in the transmission) gets you closest with .909 percentage.

The key is to always divide the smaller numbers by the larger to get the percentage and remember which way to go on the tooth count (more teeth to slow the speedometer done and less to speed it up).


This is the best way to accurately determine what speedometer gear you need. It takes into account any inaccuracies in the speedometer and the TRUE tire height (I have seen as much as 1 ½” difference in height between different brands of say 235/70/15s depending on who manufactures the tire and what rims they are mounted on). Additionally the 3 variables you need are the easiest to obtain; real speed, speedometer indicated speed and number of teeth on the speedometer gear.

On a side note the last 2 builds I did used aftermarket electronic speedometers……..man it sure is nice to calibrate the speedometer while sitting in the drivers seat and pushing a button as opposed to

Thanks for the replies everyone!

I did manage to find the tag for the front ratio under all the dirt. 2.94

I can assume the rear ratio is the same...?

It's possible to solve this with math. Multiply your gear ratio (2.94) x 262 and divide by the tire diameter in inches (33). 262x2.94/33=23 teeth, which sounds about right with that tall a tire and those gears. You'll really want some 4.10-4.56 gears for it with that tire. The equation is very precise and you can find your exact diameter by measuring the circumference and dividing by 3.1416 (aka: pi) to make it even more accurate.

texas383..dont you have a 4 link 2x truck? get a 4x4?