DougB
Roll Me Over
Reged: 11/08/02
Posts: 4123
Loc: Terre Haute, IN
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Understanding your GenI Speedometer
10/10/04 03:08 PM
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While looking for possible speedo problems last night I decided to finally pull out the gear and have a look-see.
As you can see, the assembly is comprised of the casing, the nylon gear (held in place by a spring pin), and a couple of o-rings.
The nylon gear is turned by a screw on the output shaft:
Something interesting I noticed about the speedo unit was the ancient Summarian text that the Mitsu scribes left behind:
I stumbled for a couple minutes to figure it out. You'll notice that there are four sets of numbers, each preceeded by a triangle: ^20-22, ^23-25, ^29-31, ^26-28. These numbers correspond to the number of teeth on the nylon gear on the speedo unit and are matched up with a mating mark on the outside of the transfer case:
For GenI's riding on stock tire size and final gearing of 4.625, a 26-toothed gear is used. This is how the correct speed reading displayed on the dash.
Now for people like us who have larger-than-stock tire sizes on our trucks, it will throw off the speedo reading. For myself, riding on 31" tires throws my speedometer off by 7%. To help you understand this, when my speedometer is reading 65mph, I'm actually travelling at 70mph. Unfortunately, I have a speeding ticket to verify this fact.
So, how does one correct for this error? One method that I'm sure all of us use is to just continuously mentally note to ourselves that our actual speed is higher than the displayed speed. This works okay sometimes... about as good as setting the time on your alarm clock forward by a couple minutes. 
The other method would be to change your gear size. Mitsubishi did make other nylon speedo gears for differing tire sizes and final gearing options. I've been able to identify the following:
MD705466 - Gear, Speedometer Drive (N=26)
MD705464 - Gear, Speedometer Drive (N=24)
MD705462 - Gear, Speedometer Drive (N=22)
Other parts to go with that assembly:
MF472536 - Spring Pin, Speedometer Drive Gear (3x16mm)
MD701788 - O-ring, Speedometer Drive Gear (10.3mm)
MF520403 - O-ring, Speedometer Drive Gear Casing (35mm)
(I'll dump the numbers in the Repository if this turns out successfully)
I plan on reducing my speedo gear size to the 24-toothed nylon gear. Hopefully, that will give me enough correction to avoid future speeding tickets. I haven't yet done any testing on this method, so I can't speak for its acceptability. NathanC had been in the process of scripting a spreadsheet that corresponded varying final gearing sizes and tire sizes with the correct speedo gear. I don't know if that project has been abandoned or not as he's currently got 20 irons in the fire. 
Enjoy! 
-DougB 
EDIT: Link to Pt. 2
Edited by DougB (01/12/05 04:46 PM)
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