I get 24-25mpg with my 617, which is not really strained pulling around the 126 (I also love the huge low-rev torque it has, I can cruse through the city at 1000-1300rpm with ease, even up hills)....but the 616 would be working harder, and would be rev'd higher....and you also have to remember the 3.07 diff on the SD, you'd have to swap that too.....or I doubt you'd be going very fast at all. I think you'd get Worse fuel economy with a 616 in an SD. Not to mention getting the car smashed because it can't break 50-60mph....

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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
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'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
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I dont want to hijack this thread, but I had a similar question. I have a 240D, with manual transmission(2 in fact). I was wondering about putting a turbo motor out of a 300D into my 1980 240D. Im sure that has been tried before? My oldest son wants more get up and go, but likes the manual controls throughout the 240D.
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

Easy, all you really need to do is cut the driveshaft and shift rods 4" shorter. Several people, including myself, have done it successfully.