Nice truck - but now we've got another dimension to your problem - it has to be cheap too...

As far as I know the top gear on the 4 speed transmissions (automatic or not) are all 1:1

I guess an overdrive is going to be your best option but I imagine the RPM range you want at 75 mph would only be good for a flat place (like where I live!) - a hilly place would probably cause the engine to bog down or an automatic transmission to change down. I don't know this for sure - you need to get in touch with people who have done this conversion before. I'd expect an OM617 in a truck to be revving closer to 4K at 75 mph...

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!