Modern cars with variable valve timing and other goodies can achieve very good combustion under low load. Looking around, it seems ~30km/h would be the sweet sp

Modern cars with variable valve timing and other goodies can achieve very good combustion under low load. Looking around, it seems ~30km/h would be the sweet spot. It would be slightly higher if you run the AC on high, increasing total consumption by ~10% and moving the dial a bit.