I experimented with my 350 SDL on a recent trip. One tank at 75 ran just over 3000 rpm and fuel use about 24. Slowing to 70 just under 3000 fuel use went to 25. Assuming most engines are not meant to run near max for sustained periods and as Bill notes the fuel use is kind of an indicator. The A/C on with windows closed requires the same Hp as A/C off and windows open.