I had the 2.2L for 6 years with the 5-spd (crunched by 15-yr old driving daddy's suburban . . . sniff, sniff). I never got less than an average of 37 mpg on the highway. I drove cross-country a lot. That mileage was with 195's instead of the recommended 175s which garnered the extra mileage bringing one up to 40-41 mpg (which I had for a short time). The one 2.5L turbo, with a 5-spd that I saw, and spoke to the owner about, claimed he got a steady 36 mpg. According to him, hell would freeze over before he gave up his rare car.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002