My 2001 5.9 24v Cummins with a Hypertech Stage 3 map pulls 23-24 hiway at 65-70 mph. Pulled my brother in law's 28.5 foot Alpenlite 5th wheel from Bakersfield to Sacramento, averaged 15 mpg @ 60mph, had to use cruise because it kept creeping up to 65-70 if I wasn't paying attention.

'68 Bee 383/TF/Factory Air...high school sweetheart
'67 GTX Clone project,500 six pack,Hemi4-speed,Dana
05 Dodge Viper, 505 V-10, 6-speed Tremec