False certainty to remove
- Subtracting mpg ratings instead of comparing fuel use.
- Assuming rated efficiency in every condition.
- Ignoring the financed cost of a price premium.
- Counting tax credits without confirming eligibility.
30 mpg versus 45 mpg, fuel at $3.50
Cost per mile at 30 mpg: 11.7¢
Cost per mile at 45 mpg: 7.8¢
Savings: 3.9¢ per mile
$4,000 price premium break-even: about 103,000 miles
At 12,000 miles/year: roughly 8.6 years before other differences
A range is more useful than one precise-looking forecast when an input is uncertain.
Test the assumptions that can move the answer
Use gallons per mile
Fuel cost equals miles divided by efficiency times price. Comparing mpg differences directly can mislead because mpg is reciprocal.
This decision belongs in The Real Cost of Car Ownership. Use Vehicle Cost per Mile: Use the Right Denominator to check the next assumption, then test the numbers in the Fuel Cost Comparison Calculator.
Use your driving
Annual miles, city/highway mix, charging access, climate and towing can move real consumption away from ratings.
Price volatility
Run low, base and high fuel prices. A decision that wins only at one extreme is a bet, not a robust saving.
Include the rest of the vehicle
Tax, financing, insurance, maintenance, electricity and resale can outweigh fuel savings; keep fuel as one line in total ownership cost.
Scenarios worth testing
Fuel savings recover a higher purchase price only through miles driven. Test real-world efficiency and several fuel prices, then compare the resulting break-even mileage with how long you will keep the vehicle.
Inputs for the mileage threshold
- Annual miles under your actual driving pattern.
- Realistic efficiency for both vehicles in those conditions.
- Incremental out-the-door price and expected ownership years.
- Estimate real annual miles by driving type.
- Use credible efficiency for each vehicle.
- Calculate cost per mile at several energy prices.
- Divide net purchase premium by per-mile savings.
- Check whether break-even occurs before expected sale.
Stress-test the uncertain inputs
Fuel Cost Comparison Calculator
Use gallons per mile, real annual driving and a disclosed fuel-price assumption.
Questions this decision usually raises
Why does going from 15 to 20 mpg save more than 30 to 35?
Because gallons per mile falls nonlinearly; the same mpg increase saves more fuel at low starting efficiency.
How do EVs fit?
Use energy per mile, charging price, losses and public-charging mix, then include different maintenance and resale assumptions.
Should time at fuel stops count?
Only if it differs materially and you disclose how time is valued.
Evidence and assumptions to revisit
This page explains a calculation, not a personal recommendation. Contract rules and consumer protections vary by product and jurisdiction. Check the current agreement and official guidance before acting.