Speedometer Question

I am rebuilding a dana 18 that was originally in a 4.27 rear gear equipped jeep. That transfer case is going into a 5.38 rear gear equipped jeep. The speedo drive gear is on the rear output housing of the dana 18. Does that mean I need to swap the speedo gear out to get the speedo to read accurately?

 

It will be off.....I am not sure how much.

 

i'd imagine it'll be off a good amount going from 4.27 to 5.38. thats over one full difference in driveshaft revolution.

 

It will be off 25%, which means that if your old speedo reads 50 the new set up will need to read 62 to actually do the same speed. Speedometers are never off a fixed amount but a ratio. So the faster you go the more they are off.

 

Right. So If I swap the speedo gear in the T case to one that will work with 5.38 gears, than I should be okay. Is there a particular tooth count I need to look for?

 

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A major reason it would be nice if someone made an electronic version of the jeep speedo, so it could be adjusted for any combo.

 

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Great until you are in a heavily-wooded mountain canyon on a snowy day.....

 

In which case you may not really care how fast (or slow) your going....

 

Jim, the tooth count and part numbers you mention for the 4.27 gears is exactly what I have. 18 tooth (SW448493) and 6 tooth (SW447331). I have a spare dana 18 under the workbench that I *think* came from a jeep with 5.38 gears (not sure on that). it has a 4 tooth drive gear (SW448110) and a 15 tooth driven gear (SW448110). do any of your books show these part numbers or tooth counts?

18/6=3
15/4=3.75
i havent put much thought into the math, but i think the 15/4 combo is for a lower (numerically higher) gear set than the 18/6 combo. any thoughts?

EDIT: come to think of it, the tooth count on the drive gear (either 4 or 6) shouldn't matter, its the pitch of the teeth that would matter, and as far as i can tell with the naked eye, the pitch is the same. but i still think the 15 tooth driven gear is for a lower (numerically higher) gear ratio.

 

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there is an electric speedo, takes some fabing but it will work, bicycle computer. JP did it a few years ago, not sure how well it would work in the dark( maybe a red light next to it) but they are digital, weather resistant and dont run off your electrical system.
IIRC they JB welded the sensor magnet to the back of the rear t-case yoke then made a bracket that bolted to the dana 18.

 

I have a similar problem with my Dana 300. I went from 3.54 gears to 4.10's and the speedo is obviosly off. I spoke to a guy at a speedometer shop and his recommendation is : Using the size tires you intend to run, check your existing odometer agains 10 miles worth of mile markers and let them know what the error is. They will make up a conversion gear box (I forget exactly what they call it) that can go either between the existing speedo cable and the transfer case or between the cable and the speedometer. If memory serves me correctly, the cost is between $35-40 bucks.

 

I definetly won't mix the gears. i'm either using the 6/18 (which we know is for 4.27 gears) or the 4/15. your manual doesn't show what rear gear ratio was used with the 4/15 gears and the part numbers i mentioned earlier?

 

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Perfect. Thanks for the help Jim. I'll stand a much better chance at speedometer accuracy with the 4/15 gears. Anyone have a rough estimate of the total height of a 6.00x16 tire? i'm guessing around 30"? anyone have one they can measure for me?