I just put in another instrument panel into my 1984 d150. New dash came from same year/tranny/engine truck. The problem is when the indicated speed is 55 mph, I am keeping up with traffic on rural I-5, which usually runs 65-75mph. The odometer part of the gauge is almost spot on, less than 1/20th of a mile off. Is there any way for the home mechanic to adjust the speedo reading?

If the odometer is reading correctly but the speedo is not, that would tend to indicate a problem in the speedo head itself, which cannot be adjusted. But, it's pretty uncommon for the odo to read one way and the speedo another way. Usually they're both right, both too fast, both too slow, etc., and if they're both too fast or too slow, you fix it by swapping in the correct speedo drive pinion down at the transmission end of the speedo cable. Are you sure your speedo reads too slow...and that the odo reads correctly? Check carefully!

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Too many people who were born on third base actually believe they've hit a triple.

I've never come across a speedometer/odometer that didn't agree with itself to <5%.

If you want to change the speedometer calibration w/o changing the odometer's, all you need do is slightly increase or decrease the spring tension inside the speedometer or increase or decrease the drag due to the induced eddy currents. The latter is probably easier.

I'd 1st check for something bent, missing, or not rotating smoothly (especially a missing magnet). If all that checks out, I'd probably try to increase the eddy currents just a little by bending the arms holding the magnets a *little* closer to the aluminum cup. Be conservative!

If that isn't enough, you could put some new magnets onto the arms, but that will certainly take some adjustment to get right.

If I remember correctly (I don't have my reference books here), 2000 rpm of the speedometer cable should read 60mph.