Air is thick stuff. Even a 10km/h headwind can play havoc with fuel consumption. Here’s why:
At 100km/h, aerodynamic drag accounts for about half the total resistance acting against the car’s forward progress. Battling into a 10km/h headwind is like driving into still air at 110km/h. But unfortunately the drag doesn’t go up by just 10 per cent. It’s more like 20 per cent, because drag varies with the square of speed. Twenty per cent of half the total resistance means 10 per cent higher fuel consumption.
100 squared is 10,000.
110 squared is 12,100.
12,100 divided by 10,000 = 1.21 ie a 21 per cent increase.)
In the same way, a 20km/h headwind increases drag by about 44 per cent (because 120 squared is 14,400).
At highway speeds, aerodynamic drag is maybe 50 per cent of the total load the engine is working against. That means 50 per cent of your fuel consumption on the highway comes down just to pushing the air out of the way of the car.
An initial conclusion from all this is that if you drive at 100km/h instead of 110km/h you'll save about 10 per cent on your fuel bill. Because half of the resistance the engine is fighting against will drop by about 20 per cent, and half of 20 is 10 per cent.
A second conclusion is that driving into a 20km/h headwind on the highway could cause your consumption to increase by 20 per cent - because half the resistance the engine is fighting against will increase by about 40 per cent.