A car consumes a lot of fuel at speeds below, say, 40-50km/h, consumes the least fuel at speeds between 80km/h and 120km/h, then the consumption goes up again from 120km/h onwards.
At 200km/h, it burns quite a lot of fuel. At 220km/h, it consumes fuel in huge lumps. At 250km/h, the Arabs will send you t-shirts and Christmas cards.
There are a lot of caveats involved here though; the biggest ones surrounding engine size, transmission type and traffic conditions. Bigger engines are more economical at slightly higher speeds: for example, the Lamborghini is better off at 120 than it is at 80.
Smaller engines thrive at “non-motorised” pace: a 600cc Kei car is better at 70-80km/h than it would be at 120km/h.
Automatic transmission engines may not allow short-shifting unless equipped with a manual override or has numerous ratios like the Range Rover’s 9-speed. So at low speed, it will likely be at a very low gear, possibly first or second, which is exactly when Shell and BP start awarding bonuses to employees. You may be better off maintaining 100km/h, give or take 15km/h.
Traffic conditions are fairly obvious: an open road is far better for all engine types than a clogged one. Stop-start driving triples your fuel consumption as compared to highway driving.
These factors may apply in a variety of permutations, along with other variables such as vehicle weight, aerodynamic profile, right-foot flexibility, mechanical condition, and fuel quality, to prove one point I have been saying all along: fuel economy is not an exact science.