A 20 → 25 MPG upgrade saves twice as much fuel as a 40 → 50 MPG upgrade over the same miles. Both headlines say "+5 MPG." Only one metric tells you the truth without a calculator — and it isn’t the one Americans use.
The math behind the illusion
MPG = miles ÷ gallons. L/100km = liters ÷ (km/100). The first is a rate of travel per unit of fuel; the second is a rate of fuel per unit of travel. Cost scales linearly with fuel, not with miles — so any metric where fuel is in the numerator gives you an honest view of spending, and MPG doesn’t.
10,000 miles, worked out
- 20 MPG → 500 gallons.
- 25 MPG → 400 gallons. Saves 100 gallons vs 20.
- 40 MPG → 250 gallons.
- 50 MPG → 200 gallons. Saves 50 gallons vs 40.
- 100 MPG → 100 gallons. Saves another 100 vs 50 MPG — but that’s already way past diminishing returns.
The same data in L/100km
- 20 MPG = 11.8 L/100km.
- 25 MPG = 9.4 L/100km. Saves 2.4 L per 100 km.
- 40 MPG = 5.9 L/100km.
- 50 MPG = 4.7 L/100km. Saves 1.2 L per 100 km — exactly half.
The L/100km differences match the actual gallon-savings ratio. The metric doesn’t lie about which swap saves more fuel.
Practical takeaway
The biggest gains in fleet fuel use come from replacing the worst vehicles, not upgrading the best. If you drive a 15 MPG truck 10k miles a year, swapping to 20 MPG saves 167 gallons. The same owner upgrading a 35 MPG sedan to 45 MPG saves 64 gallons. Policy and purchasing decisions should weight accordingly.
Miles and gallons in — MPG, L/100km, and cost per mile out. See efficiency honestly.