Trip fuel and trip cost
Divide trip miles by MPG to estimate gallons burned. Multiply those gallons by the current price per gallon.
trip cost = miles ÷ MPG × price per gallon
Annual fuel cost
Use expected annual mileage instead of trip distance. This makes it easy to compare a daily driver, commute change or a more efficient vehicle.
Why a small MPG change matters more at low MPG
Going from 10 to 15 MPG saves more fuel per mile than going from 30 to 35 MPG. MPG is not linear when viewed as fuel consumed per fixed distance.
Use realistic inputs
EPA ratings can be useful reference points, but your actual MPG depends on traffic, speed, climate, payload, towing and driving style. For budgeting, use your own observed average when possible.