Consumption is the inverse of efficiency
US gallons per 100 miles equals 100 divided by US MPG. A 20 MPG car uses 5 gallons for 100 miles. A 25 MPG car uses 4. A 40 MPG car uses 2.5, and a 45 MPG car uses about 2.22.
Equal MPG gains do not save equal fuel
Moving from 20 to 25 MPG saves 1 gallon per 100 miles. Moving from 40 to 45 MPG saves about 0.28 gallons over the same distance. Both changes add 5 MPG, but the first saves more fuel. At 12,000 miles a year, those savings become 120 gallons versus roughly 33.3 gallons.
Turn the difference into dollars
At an illustrative $3.50 per gallon, the first change saves $420 per year and the second about $117. This calculation isolates efficiency. It does not imply that replacing a vehicle is economically justified; purchase costs can overwhelm fuel savings.
Use consistent units
MileLedger uses US gallons. Imperial MPG is a different unit. To convert US MPG to liters per 100 kilometers, divide approximately 235.2146 by MPG. At 30 US MPG the result is about 7.84 L/100 km.