Twin Speedo Transplant

[Mark DiSilvestro]

Well, now that I own 3 diesels, I need to fix speedometer problems in two of them. Both problems are the result of previous owners replacing the speedometers with incorrect ones. With 'Beirut Taxi', my '81 240D, the seller replaced the failed original 85 mph speedometer with a 100 mph unit. While the seller insisted it was the correct replacement, when I took the car to Virginia Beach, I verified with the highway mile markers that it reads about 15% low.
As luck would have it, 'Wonder Wagon', my '84 non-turbo Euro 300TD apparently had it's original kmph speedometer replaced with a 100 mph unit that reads about 12% high - verified by comparing the distance reading from College Park, MD to Fort Belvoir, VA on my '98 Nissan Altima (31 miles) with the 300TD (35 miles). I suspect a unit for a US '84 3.07 axle ratio 300TD turbo was installed in my wagon.

I've collected several used speedometers, including an original 240D speedometer that should work with the 3.69 axle ratio in Beirut Taxi, but how to tell which of the others might be compatible with Wonder Wagon's 3.46 axle ratio. (One of those came out of an '83 300D turbo so I'm sure that one won't work)

Is there a sticker, decal or some other marking on these VDO speedometers that indicates what vehicles or axle ratios they're designed for?
If not, I suppose I could swap the low-reading unit from Beirut Taxi into Wonder Wagon and see what happens.

Happy Motoring, Mark