I regularly get 18.6 driving back and forth to work (100 mi per day 60 Hwy/40 City). On the open hwy, the mileage is directly related to speed.
Spd/Mpg
60/24
65/22.6
70/21
75/20.2
80/18

The truck is 100% stock 2002 DA 2500HD CC LB. I run AmsOil 15w-40 synthetic oil and a OEM style K&N air filter.

Mike .T.