Set a monthly fuel budget.
A fuel budget is built from mileage rather than from what you happen to spend at the pump, because tank fills are irregular and mislead you about the underlying rate. Weekly commuting is converted to a monthly figure using 4.345 weeks per month rather than four, which matters — using four understates the annual total by about 8%, or a month's fuel. Add discretionary mileage separately, divide by economy for gallons, and multiply by price. Keep the cost per mile visible: it is the number that lets you price any individual journey instantly and is unaffected by how much you drive.
Monthly Fuel Budget
Monthly miles = weekly commute x 4.345 + other monthly miles; gallons = miles / mpg; budget = gallons x fuel price; cost per mile = price / mpg
Because a year has 52.14 weeks, not 48. Multiplying weekly mileage by 4 loses about a month of driving a year, which is a material error in an annual budget.
Add 10-15% or use the highest price you have paid in the last year. Pump prices commonly swing that much within a season, and a budget set at today's low is one that fails in summer.