T18 Speedo Drive Gear Color ?

[Canuck]

There are 3 drive gears (on the output shaft) available for the T18: Green, yellow and brown.

Which is the correct one for a 3.73 diff ?

I had a green one but it's had it's teeth worn off from wear or something being wrong. Given the history of this truck (she's a bit of a mutt) I can't be sure that the green one is even the right one. I'd like to make sure before I go through the trouble of replacing it.

[robroy]

Good day Canuck,

I can't be certain that it's the correct one, yet here's the gear from my 1972 F250 w/T18 and a 3.73 Dana 60.




With this speedometer gear installed, the speedometer seemed about right (at least based on having looked at a few roadside automatic speed indicators over the years).

Robroy

[Canuck]

Thanks Robroy, but it's the gear INSIDE the tranny (the one that the gear in your photo meshes with) that I'm checking on. That being said, it's good to know that the driven gear should be a 19 tooth !

[robroy]

Good afternoon Canuck,

Ah ha! I thought there was something funny going on here, since my gear didn't exactly match any of those colors you mentioned. Thanks for the explanation.

Robroy

[DMiller]

Hi Canuck,
I have 71 F250 -Dana 4.10/ T18 with yellow drive gear.

[Canuck]

ok...so it looks like it's not the yellow -which is too bad because I actually have a usable yellow gear

[70_F100]

Need to know tire size and rear end gearing to detemine the correct gears.

Since we already know the gearing, if you'll let me know the tire size I can tell you what you need.

[ultraranger]

http://www.gtsparkplugs.com/SpeedoGearCalc.html

Once you get the speedometer working, here is a method you can use to determine its accuracy:

Use or borrow a GPS that reads actual MPH. Check your tire pressure. Go for a drive on an open flat road where you can hold the speed exactly. Pick a safe speed - faster is more accurate, but any speed - and hold it. Read the GPS and drive long enough to get solid speedo and GPS speeds.

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


Let's say you drove at 47mph on your speedo, and the GPS said 44. Let's also say you have a yellow 18-tooth gear. The formula would be: (18 x 47)/44 = 19.23 teeth

[Canuck]

Need to know tire size and rear end gearing to detemine the correct gears.

Since we already know the gearing, if you'll let me know the tire size I can tell you what you need.

235/85/16. This is the stock size for the truck.

[70_F100]

Need to know tire size and rear end gearing to detemine the correct gears.

Since we already know the gearing, if you'll let me know the tire size I can tell you what you need.

235/85/16. This is the stock size for the truck.

You need a 7-tooth drive gear and 17-tooth driven gear.

[Canuck]

ok -great !

Now....how do you know that ? Is there a chart somewhere ?

If this is simply a ratio then presumably a different combination of gears could be used ?

Any idea what happens if they drive gear has the wrong number of 'teeth' ?

[Dalton Sexton]

How do you know how many teeth inside the transmission? I am making a custom gauge cluster and my current speedo cable is toast and it's gear is stripped so I must purchase a cable and gear. I have 31 x 10.5 x 15 tires. The rear end is 3.25.

[Ranchero50]

How do you know how many teeth inside the transmission? I am making a custom gauge cluster and my current speedo cable is toast and it's gear is stripped so I must purchase a cable and gear. I have 31 x 10.5 x 15 tires. The rear end is 3.25.

Get it working again, then run 10 miles down the interstate and compare your odometer to the mileage markers on the side of the highway. Over a couple miles it will average out and you know how far off your measured distance is to the actual distance. If you measure 1.1 miles per 1 mile actual they you need a driven gear that's 10% larger. To calibrate the speed, just use a GPS stopwatch and see how long it takes to cover one mile at 60 mph. Should be one minute per mile.

The only time I've seen gears stripped is when someone installed a counter clockwise gear on a clockwise setup or vice versus. Ford used to sell both and I've sure they are still available out there.

[Dalton Sexton]

Thanks!