I've only been able to accurately take mileage on my diesel suburban a couple times since I bought it (last August). On my winter fuel mix (I believe it is 70%D2 and 30% Kero around here) I got almost exactly 16 mpgs (Miles driven / gallons of diesel to fill up) of mixed city freeway with light loads and no trailer.

I've never driven an Excursion so I can't speak for them. I would say the largest differences are that the Excursions are newer. Had I had another 10k to spend I would have bought and Excursion. But I only wanted to spend a few grand in cash so I bought a Suburban. I had to replace the IP because the lift pump went out and was not supply proper fuel pressure to the IP. I got a great deal on the IP and put it on myself so it wasn't too much damage. I also have not towed anything heavy with it but so far I love it. I grew up with Suburbans (one diesel and the rest gassers, my Dad still has an 06 Burb).

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD