Start with two actual configurations
Compare vehicles that meet the same needs. A larger hybrid with all-wheel drive and a smaller gasoline car are not interchangeable purchases. Match the model year, drivetrain and equipment as closely as practical, then use the exact EPA configuration or your measured MPG.
Separate the two calculations
First estimate annual energy cost for each car. A 30 MPG car driven 12,000 miles at $3.50 per gallon costs $1,400 per year. A 40 MPG car with the same distance and price costs $1,050. The modeled fuel saving is $350 per year. These are illustrative assumptions, not quoted vehicle prices or current fuel averages.
Divide the premium by the saving
If the more efficient car costs $3,000 extra, simple fuel-only payback is $3,000 ÷ $350, or about 8.6 years. With no positive annual fuel saving, there is no fuel-only payback. At 6,000 miles per year the annual saving halves, so the same premium takes twice as long to recover.
A purchase decision needs more than payback
The calculation excludes depreciation, resale value, maintenance, financing, insurance, taxes and time value of money. It also assumes constant prices and usage. Use the result to test whether fuel savings are material to your situation, not as a personalized recommendation to buy a vehicle.