Fuel saved goes as the difference of the RECIPROCALS. Taking a 10 mpg vehicle to 20 saves 5 gallons per 100 miles; taking a 30 mpg car to 60 saves 1.7 — the same doubling, a third of the fuel.
Distance-per-fuel and fuel-per-distance are reciprocals, so equal steps in one are not equal steps in the other. What you actually buy is fuel, which means the honest unit is gallons (or litres) per fixed distance, and in that unit the same improvement is worth the same wherever it happens. In miles per gallon it is not: the curve is steep at the bottom and almost flat at the top, so the biggest savings hide at the unglamorous end. Fix the worst vehicle in a fleet before perfecting the best one. US window stickers have printed gallons per 100 miles alongside mpg since 2013 for exactly this reason.
It is arithmetic about fuel, not about money or carbon in every case — fuels differ in price and in carbon per litre, and an electric replacement leaves this scale altogether. And it assumes the same distance is driven either way, which is the thing rebound effects tend to undo.
Estimate with the rule, then check it against the calculator that models it properly.
Open Unit Converter →Fuel saved goes as the difference of the RECIPROCALS. Taking a 10 mpg vehicle to 20 saves 5 gallons per 100 miles; taking a 30 mpg car to 60 saves 1.7 — the same doubling, a third of the fuel. Distance-per-fuel and fuel-per-distance are reciprocals, so equal steps in one are not equal steps in the other.