Gas is the one road-trip cost you can predict almost exactly, because it comes down to three numbers you already have: how far you are going, how far your car goes on a gallon, and what the pump is charging. The arithmetic is two steps, and the calculator above shows both. The figures below use its default trip so you can check the method against numbers you can see. Counting down to the trip? The date calculator gives the days between today and departure, and the auto loan calculator covers the payment under the car itself.
A worked example: 480 miles at 28 MPG
A 480-mile drive in a car rated at 28 MPG needs 480 ÷ 28 = 17.14 gallons. At $3.78 a gallon that is 17.14 × $3.78 = $64.80 of gas, or about 1.1 fill-ups of a typical 15-gallon tank. Dividing the price by the MPG gives the figure that travels best between trips: $3.78 ÷ 28 = $0.135 a mile, which is $13.50 per 100 miles. Switch on the round-trip option and both distance and cost double — 960 miles and $129.60 — while the cost per mile stays at 13.5 cents, because it does not depend on how far you drive.
Trip cost example: 300 miles at 25 MPG
Here is a second trip with round numbers. The $3.50 gas price is only an example, not a current or average pump price, so enter what your local station charges. A 300-mile drive in a car that gets 25 MPG uses 300 ÷ 25 = 12 gallons (12.0 gal in the result panel). At $3.50 a gallon the fuel costs 12 × $3.50 = $42.00, and the cost per mile is $3.50 ÷ 25 = $0.140, or 14 cents a mile and $14.00 per 100 miles. Make it a round trip and the drive doubles to 600 miles, 24 gallons and $84.00, while the cost per mile stays at 14 cents.
What cost per mile tells you
Total trip cost answers one question; cost per mile answers every other one. It is the number to compare two cars with, to decide whether a detour is worth it, and to check a mileage reimbursement against what the drive actually costs you in fuel. At $3.78 a gallon, a 28-MPG car runs at 13.5 cents a mile, a 20-MPG SUV at 18.9 cents, and a 45-MPG hybrid at 8.4 cents — so over 15,000 miles a year the gap between the SUV and the hybrid is more than $1,500 in gas alone. Because it is just price divided by MPG, cost per mile moves the moment either one does.
Knowing your real MPG
The window-sticker MPG is an average under ideal conditions. Your real number depends on speed, terrain, cargo, tire pressure, and how aggressively you accelerate. For the most accurate estimate, calculate your own MPG: fill the tank, reset the trip odometer, and at the next fill-up divide the miles driven by the gallons it took to refill. Use that figure here instead of the factory rating for trip planning you can trust — a car that returns 25 MPG rather than its rated 28 adds about $7.80 to the 480-mile trip above.
Highway versus city driving
Most vehicles are noticeably more efficient on the highway than in stop-and-go city traffic, where constant braking wastes energy. The US Department of Energy and the EPA put the cost of aggressive driving — speeding and hard acceleration and braking — at roughly 15% to 30% of your gas mileage at highway speeds and 10% to 40% in stop-and-go traffic, and note that mileage usually falls off rapidly above 50 mph. A long interstate trip may beat your car's combined rating, while a day of errands around town falls below it. If your journey mixes both, use the combined MPG, or split the trip and calculate each leg separately for a sharper estimate. For distances listed in kilometers, convert them first with the length converter.
Cutting your fuel costs
Small habits add up over thousands of miles. Keeping tires properly inflated, removing roof racks when unused, easing off hard acceleration, and combining errands into one trip can each improve efficiency by a few percent. On longer drives, using cruise control on flat highways and sticking near the speed limit often saves more fuel than any single mechanical fix: the Department of Energy and EPA reckon each 5 mph above 50 mph costs you about the same as paying an extra $0.31 a gallon. Financing the car itself? The auto loan calculator estimates the monthly payment.