I just found some old records for trips I took on my 350.
One in particular was a trip from Iowa to Yellowstone Natl Park, down through Rocky Mountain National Park, and back to Iowa.
This is 2 up, pulling a small trailer, mostly highway driving.
Total miles driven: 3228
Gal gas used: 74.8
Average miles/gal: 43.2
Qts oil used: 13
Miles/qt: 248