I have a 1982 300SD with about 280K miles. The diff had insane amounts of backlash, so I picked up a nice tight used unit from a junkyard for $100 and had it installed (along with new oil seals and rebuilding the CV joints on the axles and flex plate on the driveshaft, as long as it was all out...)

Silly me, I believed the junkyard man who said it was from a 300SD, and I didn't open it and count teeth before having it installed. Now my speedometer is off! Marking the wheel and driveshaft and counting revolutions, it looks like I ended up with a 3.69:1 diff instead of the stock 3.07:1. That mismatch seems to correlate well with the speedometer error at different speeds. Sigh.

Because of the expense, and because it's kind of nice to get better acceleration and we don't have so much need for low RPMs at highway speeds, I think I'm going to keep the diff and fix the speedometer.

How were the speedometers changed to match the different axle ratios on the various models of cars?
-- completely different speedometer
-- internal alterations to same speedoemeter design
-- different transmitter ring or gear in transmission
-- something else entirely

I'm trying to avoid buying one of those $140 generic electronic ratio adapters. Surely there is some transmission part I can buy? Or resistor inside the speedometer I can change? I have a spare speedo to play with, if it comes to it.

Thanks,
Duncan

 

Duncan-- I had the same problem when I swapped the rear axle from my parts car, a 1960 gas 190, into my 1959 180-D, to cure an insanely-loud howl. Same thing -- the diesel had a 3.7:1 rear end, the 190 had a 4.3:1, and the speedo and odometer were accordingly 'way off. I fixed it by swapping speedos, swallowing the unrealistic top end and shift points on the 190 unit.

Whatever you do, you're going to want it to work on both the odometer and the speedo, and I don't think there's an electronic whatchamacallit that's going to do that. There was nothing electronic about either of the speedos I was working with, but there was a worm gear drifing the odo which might have been swap-able. I suspect you'll have to swap the whole speedo unit, though -- everything was accessible in these 1960 jobs once you took the speedo out of the dash, but people have gotten a lot more hard-line about protecting the integrity of milage recorders since then; I doubt you can open them up and get into the inner workings any more.