Fuel cost calculator
A 500-mile road trip in a 25 MPG car at $3.50/gallon costs $70 in fuel. The same trip in a 35 MPG car costs $50. Over 15,000 miles/year, that 10 MPG difference saves $600 annually. When gas prices spike to $5/gallon, the annual savings jump to $857. Fuel efficiency isn't exciting until you multiply by miles driven.
Good to know
Highway vs. city MPG matters. EPA ratings list both: highway is typically 20-30% better than city. A car rated 28 city / 35 highway will average around 30-31 MPG on a mixed road trip. For pure highway driving, use the highway number. For urban trips, use city MPG.
Speed kills efficiency. Most vehicles achieve optimal MPG around 50-55 mph. Every 5 mph above 50 is like paying an extra $0.20-$0.30 per gallon. At 75 mph vs. 55 mph, you might lose 15-25% of your fuel efficiency. Slowing down saves money; highway speed limits aren't set for efficiency.
EVs flip the calculation. Electric vehicles measure in kWh per mile, not MPG. At $0.15/kWh and 3 miles/kWh, a 500-mile trip costs $25 in electricity. That same trip in a 25 MPG gas car at $3.50/gallon costs $70. EVs are cheaper to fuel in most markets, though purchase price and range considerations complicate the comparison.