mileage
Don't know if this is your first diesel or not-- but I would suggest doing a search. I have found several TDR threads with some really good input from very experienced drivers. Here's one:
MPGstinks,ramman2u
Basically, it comes down to: rear axle, acceleration habits, highway cruising speed, and manual vs. auto transmission:
3. 54 axels will average better mileage than 4. 10s-- all else being equal.
Acceleration: If you're after better mileage, you can't drive these like gas engines. With diesels, use your right foot like there was an egg under your shoe (got that from Dl5treez).
Highway: fuel efficiency drops precipitously with speed. Soon after buying my truck, I ran the same 1200 mile round trip twice (2400 mi total) and was very careful about speed to see what would happen- once cruising at 60 mph, and once at 68-70-- the mileage was 23. 45 and 18. 75, respectively.
A manual transmission transfers engine torque through an entirely mechanical linkage, and so is 100% effecient. A factory auto transmission typically locks up only at higher speeds-- this means that when you're driving around town it is not locked-up and there is a fluid "gap" in the drive train: the torque converter is using fluid pushed by the impeller to move the stator-- and this means less than 100% efficient transfer of torque. I've never had a manual cummins, but I've read this often translates to only about 1 mpg or so.
I typically get around 18-19 in town, and around 22+ on hwy-- and I couldn't be more pleased considering the size and power of these CTD trucks!