The Speed That Costs You Twice
Drive at 110 km/h instead of 130 km/h on a motorway and most people expect to save fuel roughly in proportion — maybe 15%, matching the speed reduction.
The actual saving is usually considerably larger, and the reason is that the dominant force opposing your car at motorway speed doesn't scale with speed. It scales with speed squared.
The Two Forces
At any steady speed, your engine is overcoming two main resistances.
Rolling resistance
Tyre deformation, bearing friction, driveline losses. Approximately:
F_rolling = C_rr × mass × g
Roughly constant with speed. A heavier car has more of it; a faster car doesn't, at least not much.
Aerodynamic drag
Pushing air out of the way:
F_drag = ½ × ρ × Cd × A × v²
Where ρ is air density, Cd the drag coefficient, A the frontal area, and v the speed.
That v² term is what changes everything. Double your speed and drag force quadruples.
Where the crossover sits
At low speeds, rolling resistance dominates. At high speeds, aerodynamic drag does. For a typical passenger car the crossover falls somewhere in the region of 60–80 km/h, depending on the vehicle's shape and weight.
Below that, city driving efficiency is mostly about mass, stop-start losses, and idling. Above it, it's almost entirely about air.
Why Power Scales With the Cube
Here's the part that makes high-speed cruising expensive.
Power is force times velocity:
P = F × v
Since drag force goes as v², drag power goes as v³.
100 km/h → 130 km/h is 1.3× the speed
Drag force: 1.3² = 1.69×
Drag power: 1.3³ = 2.20×
You need more than twice the power at the wheels to overcome air resistance at 130 than at 100.
Fuel consumption doesn't rise by the full 2.2× — you're covering the distance faster, and rolling resistance and engine efficiency both complicate the picture. But the direction and rough magnitude hold, which is why motorway fuel economy degrades sharply above about 110 km/h.
The distance adjustment
To be careful about the arithmetic: what matters for fuel economy is energy per kilometre, not power.
Energy per km = drag force (which goes as v²)
So drag energy per kilometre scales with the square of speed, not the cube. Going from 100 to 130 km/h increases the drag component of your per-kilometre fuel use by about 69%, not 120%.
Since drag is most but not all of your consumption at that speed, the real-world change in L/100km is typically in the range of 20–40% depending on the vehicle. Still substantial, and still far more than the 30% speed increase would naively suggest.
The Units Problem
This is where fuel economy comparisons go wrong, and it's the same reason the drag numbers are counterintuitive.
L/100km is a consumption figure. Lower is better. It's linear in fuel used — going from 8 to 6 L/100km saves exactly 2 litres per 100 km.
MPG and km/L are efficiency figures. Higher is better. They're the reciprocal of consumption, which makes them nonlinear in fuel saved.
That reciprocal relationship is why MPG comparisons mislead. The Fuel Economy Converter handles the conversion:
- Enter the value.
- Select the source unit — MPG (US), MPG (imperial), L/100km, or km/L.
- Read the equivalents.
Note the two different MPG gallons. A US gallon is about 3.785 litres; an imperial gallon about 4.546. An imperial MPG figure is roughly 20% higher than the US figure for the same car. Comparing a UK review to a US one without converting produces a car that looks 20% more efficient than it is.
Speed and Consumption Worked Through
Take a car achieving 6.0 L/100km at 100 km/h, where roughly 65% of the energy at that speed goes to overcoming drag.
At 130 km/h, the drag component scales by 1.3² = 1.69:
Drag portion: 6.0 × 0.65 = 3.9 → 3.9 × 1.69 = 6.59
Non-drag portion: 6.0 × 0.35 = 2.1 → roughly unchanged
New total: ~8.7 L/100km
That's a 45% increase in consumption for a 30% increase in speed. On a 600 km journey:
- At 100 km/h: 36 litres, 6 hours
- At 130 km/h: 52 litres, 4.6 hours
You save about 84 minutes and spend an extra 16 litres. At €1.70/litre that's around €27 for the time saved — whether that's worth it is a personal call, but it's a much larger number than most people assume.
These figures are illustrative; the exact split between drag and rolling resistance varies considerably between vehicles.
What Else Moves the Number
Frontal area and drag coefficient. These multiply together in the drag equation, so Cd × A is the meaningful figure. A tall SUV with a good Cd can still have worse drag than a low saloon with a mediocre one, simply because of frontal area.
Roof boxes and bike racks. A roof box can add substantially to Cd × A, and the penalty scales with v² like everything else — so it costs little around town and a great deal on a motorway. Remove them when not in use.
Open windows versus air conditioning. At low speed, open windows are cheaper. At motorway speed, the added drag from open windows generally exceeds the compressor load, so air conditioning wins. The crossover is usually somewhere around 70–90 km/h.
Air density. Cold, dense air produces more drag than warm air. Altitude reduces it. This is a real but modest effect compared to speed.
Tyre pressure. Under-inflation raises rolling resistance, which matters most at lower speeds where rolling resistance dominates.
Mass. Affects rolling resistance and, more importantly, the energy needed to accelerate. In stop-start driving, mass is a major factor; at steady motorway speed, much less so.
Headwind. The drag equation uses airspeed, not ground speed. A 20 km/h headwind at 110 km/h ground speed means 130 km/h of air being pushed aside — with the v² penalty applied to that higher number.
Practical Tips
The single biggest lever on a long motorway journey is speed. Nothing else you can do on the day comes close.
Remove roof accessories when not in use. The penalty is entirely wasted if the box is empty.
Compare consumption figures in L/100km. It's linear, so differences mean what they appear to mean.
Check which gallon a MPG figure uses. US and imperial differ by about 20%.
Don't over-index on mass for motorway driving. It matters for acceleration and city driving; drag dominates at speed.
Keep tyres correctly inflated. Cheap, easy, and it helps most in the conditions where drag helps least.
FAQ
Why does drag rise with the square of speed? Because you're pushing more air per second and pushing it harder. Both effects scale with speed, so the force scales with speed squared.
Does fuel consumption really rise with the cube of speed? Power does. Fuel per kilometre scales with the square, because you're covering the distance faster. The per-kilometre figure is what you feel at the pump.
What's the most efficient cruising speed? For most cars, somewhere in the 70–90 km/h range, where drag hasn't yet dominated but the engine is operating reasonably efficiently. Higher gears and lower speeds beyond that point start to hurt engine efficiency.
Windows or air conditioning? Windows at low speed, air conditioning on the motorway. The crossover is typically around 70–90 km/h.
Why do US and UK MPG figures differ? Different gallons. An imperial gallon is about 20% larger than a US gallon, so the same car scores about 20% higher in imperial MPG.
Does this apply to electric vehicles? The aerodynamics are identical. EVs are affected the same way — motorway range drops sharply at high speed, which is why EV efficiency figures degrade faster on motorways than in town, the opposite pattern to petrol cars.
The Takeaway
Air resistance is the reason the last 20 km/h of motorway speed costs so much more than the first. The v² relationship means small speed reductions produce disproportionate savings — and it's also why a roof box you forgot to remove is quietly expensive on every long trip.
Convert between MPG, L/100km and km/L free with the Fuel Economy Converter at sadiqbd.com — no sign-up, instant results.
