Find the most fuel efficient speed.
Fuel economy peaks somewhere near 50 to 55 mph for most cars and falls away on both sides. Above that, aerodynamic drag grows with the square of speed and power demand with the cube, so economy drops roughly 1.6 percent per mph — the reason a 75 mph cruise costs about a third more fuel than 55. Below it, economy also declines, but gently, as the engine spends more time outside its efficient load band. Pairing the extra fuel cost against the time saved is the actual decision: on a 300-mile trip the trade is usually an hour against a few currency units.
Economy at speed
mpg = mpg at 55 x (1 - 0.016 per mph above 55, or 0.006 per mph below)
Trade-off
Extra cost = trip cost at your speed - trip cost at 55; time saved = miles/55 - miles/speed
Close, but it depends on gearing and body shape. Modern cars with tall top gears often peak between 50 and 60 mph; tall vans and cars with roof loads peak lower because drag bites earlier.
Financially it is a modest saving for a real time cost, and this calculation shows both sides. The stronger argument for the 55 to 65 band is that it also reduces stopping distance and crash energy.