hugho

I have 2 old W123 cars. a 1982 240D with a MT and an '85 300D. Both wonderful cars. The 85 will cruise quietly at 65 and get up to 32 mpg with a 2.9 dif.. The 240 at the same speed gets 34-36 mpg with the engine screaming because of the 3.69 dif. After many many years I decided to try out swapping a 3.07 from an '84 300D which only had 135K miles. Easy swap into the 240D with my car lift and a transmission jack. Just got it on the road. Acceleration is slower of course but engine noise very quiet at 65 due to the reduced revs by 15-20% I guess. Has anyone else tried this swap? I am expecting increased mpg but do not know by how much. I went to tall narrower tires which lowered the rear end ratio on my old 12 valve Cummins and fuel mileage went up from 20-21 to 22-23 mpg. about 10%. I am expecting a similar increase in the 240D. The electronic fuel control of newer cars generally prevents increases in economy by such simple changes as a dif modification but I am hoping for a jump in economy albeit with somewhat reduced acceleration. Has anyone tried this?

Mike P

Hugho:

I have a 1982 240D with a stick........I put in a 2.88 several years ago. Some of the forum members expressed severe doubts. I calculated required HP( using drag coefficients and estimated power at 60MPH) and engine HP produced at various RPMs. It worked on paper and more importantly in actuality!!! Slower initial acceleration but within reason.......it was never fast anyway. Quieter highway cruising and slightly better fuel mileage.

Best regards
Mike

hugho

Thanks Mike. I wouldn't know a drag coefficient from a drag queen but I like it so far now that I can listen to the radio at 60 mph for once. I guess I was hoping for mpg's approaching my TDI's in the 40's but the VW's are all electronified and light with low drag etc but I'll take an 240D anyday while I wait for the supply lines drying up.. I'll post when I get a chance on the MPGs once I get the speedo calibrated and get a chance to take a trip. I don't know where on the RPM curve is the lowest consumption and no tach anyway. On the VWs it was 2000 rpm and the cummins trucks it's 1650 and that corresponds to road speeds of about 55 or 60 . No interstates around here and that is our normal road speed. IO'd be terrified trying to enter an interstate entrance ramp anyway.

Mike P

Hugho:

You mentioned calibrating the speedo......I guess it would be possible to alter the internal gearing within the unit but I swapped out the whole instrument cluster for a matching 2.88 unit( with nearly identical mileage)......with a tach. The activation of the tach does require some minor mechanical modifications.

My 240D turns about 2350RPM@60MPH........the slow car becomes even slower off the line. After it's rolling the acceleration rate is very similar to a 3.46 car.

Great mod!!

Best regards
Mike