I used to have a 454 propane/gas convertable in my '78 4x4, 1/2 ton pickup (no overdrive).
80 gallons of propane and two tanks of gasoline (total of 117 gallon) would carry me to Omaha and back, which was well over 1,000 miles.
Had to fill up 30 miles from home on the return. Figured out to be just under 9mpg, which I thought was great for that old guzzler.
454s are drinkers, ya know. :>)