We used bob tails to deliver gasoline until the mid 70's. The engines reached their rated HP at 4200 to 4400 RPM. They were happy at that speed as long as we had a good cooling system. But they guzzle gas. That is the nature of the beast. We didn't expect over 5 miles to the gallon. My first semi was a V8 gas burner. I never let it get under 3800 and it was the happiest at 4200. Grossing 72,000 I got less than 2 miles per gallon. My line haul trucks now have 10.6 liter Cummins that cruise at 1500 RPM and get a little over 6 miles to the gallon grossing 84,000. Don't expect to approach modern performance and modern fuel economy with 45 year old engineering.
