Why lower usually wins, and where that stops
Every revolution costs friction and pumping work before it moves the car, so fewer revolutions per mile is the right default: it is why overdrive gears and tall final drives exist, and why fueleconomy.gov's efficient-driving guidance favors reaching higher gears smoothly and holding moderate speeds fueleconomy.gov2026-08. The exception is load: at very low RPM under high demand, the engine runs high cylinder pressures inefficiently and risks knock (lugging). Specific fuel consumption is best in a mid band under moderate load, typically translating to roughly 2,000 to 3,000 rpm cruising in petrol cars and lower in diesels.
Gearing sets your cruising RPM, not your right foot
example: 70 mph in 6th, 0.75 gear ratio, 3.9 final drive, 205/55R16 tire
tire dia ≈ 632 mm; circumference ≈ 1.985 m
wheel rpm = 70 mph = 112,654 m/h ÷ 1.985 m ÷ 60 ≈ 946
engine rpm = 946 × 0.75 × 3.9 ≈ 2,767
That arithmetic is the RPM to mph converter run backward: at a fixed road speed, the engine turns where the gearing puts it. A taller top gear is the only way to cruise the same speed at lower RPM, which is precisely what manufacturers add overdrive ratios for.
What moves fuel economy more than RPM
- Speed itself: aerodynamic drag grows with the square of speed; fueleconomy.gov singles out high-speed cruising as a major consumer fueleconomy.gov2026-08.
- Acceleration style: hard launches spend fuel the cruise never gets back.
- Idling: zero miles per gallon by definition; long stationary idling is pure cost.
- Load and tires: roof boxes, underinflation and dead weight tax every revolution.
Chasing the perfect cruising RPM is worth a few percent; the habits above are worth multiples of that. Shift technique, the one RPM lever genuinely in your hands, is covered on best RPM to shift.