β½ Fuel Consumption Calculator
MPG, L/100km, and trip fuel cost from your miles, gallons, and gas price.
Fuel Economy
How to Use the Fuel Consumption Calculator
Enter the miles you drove and the gallons it took to refill the tank, then press Calculate. You get MPG and L/100km instantly. Add a trip distance and your local gas price to see the trip fuel cost and gallons needed. For the most accurate MPG, use full-tank to full-tank measurements over two or three fill-ups rather than a single partial fill.
The honest way to measure: fill the tank completely, reset the trip odometer, drive normally, then fill completely again. Miles on the trip meter divided by gallons on the pump receipt is your real MPG. Do it three times and average; single-tank figures bounce around with pump shutoff differences.
Formulas
The two rating systems run in opposite directions, which confuses cross-border car shopping. US MPG rewards bigger numbers; European L/100km rewards smaller ones. The constant 235.214 converts between them exactly. Trip math is simple division and multiplication, but running it before a road trip replaces fuel anxiety with a concrete budget number.
Worked Example
You drove 300 miles and the refill took 10 gallons. MPG = 300 Γ· 10 = 30 MPG. In metric: 235.214 Γ· 30 = 7.84 L/100km. Planning a 400-mile trip at $3.50/gallon: gallons needed = 400 Γ· 30 = 13.33, cost = 13.33 Γ 3.50 = $46.67. Cost per mile = 3.50 Γ· 30 = 11.7 cents. These four numbers describe your fuel economy completely.
What Good MPG Looks Like in 2026
Compact gas sedans now commonly hit 35+ MPG combined, hybrids push 45-55, and efficient EVs rate 110-130 MPGe. Midsize SUVs sit around 25-30 MPG gas or 35-40 hybrid. Full-size trucks manage 17-23 MPG. If your car beats its EPA combined rating, you are driving efficiently; matching it is normal; trailing by 25% suggests habits or maintenance worth addressing.
Context matters more than the raw number. A 20 MPG truck that hauls tools daily is doing its job; a 20 MPG compact sedan driven gently has a problem. Compare against the EPA rating for your specific vehicle first, then against the class average. The calculator cost-per-mile figure is the great equalizer for budgets: it turns any MPG into dollars.
Driving Habits That Move the Needle
Smooth driving is the biggest lever. Jackrabbit starts and hard braking can waste 30% on the highway and 40% in town. Anticipate stops, accelerate moderately, and keep highway speed near 65 mph. Cruise control helps on flat roads but can hurt in hills, where letting speed drift slightly saves fuel versus the system downshifting to hold speed.
Cut idle time: modern engines need no warm-up beyond 30 seconds, and restarting uses less fuel than idling past a minute. Combine short trips, since a cold engine can use double the fuel for the first few miles. Remove roof racks when not in use; at highway speed a cargo box can cost 10-25% in drag. These habits compound into real money over a year.
Maintenance That Protects MPG
Tire pressure is the classic: every 1 PSI under inflation costs about 0.2% in fuel, and most cars run several PSI low. Check monthly when tires are cold. A dirty air filter, worn spark plugs, and dragging brakes all steal economy silently. Sticking to the manufacturer oil grade matters too; thicker oil adds internal drag.
Watch for sudden MPG drops as a diagnostic. A 15% overnight fall often means a stuck brake caliper, a failing oxygen sensor, or a thermostat stuck open keeping the engine cold. Track MPG per tank in a notes app; the trend line catches problems months before they strand you. The calculator cost-per-mile makes the savings from each fix concrete.
Hybrids, Plug-ins, and MPGe
Hybrids earn their MPG by keeping the gas engine in its efficient zone and recovering braking energy. They shine in city driving where conventional cars waste the most: idling, creeping, and stop-start cycles. On steady highway cruises the advantage shrinks, since the engine runs directly much of the time. Match the technology to your driving mix.
Plug-in hybrids add a wrinkle: MPG looks infinite on electric miles and ordinary on gas miles, so track both. MPGe converts electricity to gasoline-equivalent energy (33.7 kWh = 1 gallon) for comparison. A car rated 110 MPGe uses energy like a 110 MPG gas car, but your cost per mile depends on local electricity versus gas prices. Run both numbers.
Fuel Grades and Additives
Higher octane does not mean more energy; it means more knock resistance. In an engine designed for regular, premium fuel buys nothing but a lighter wallet. In an engine that recommends premium, using regular forces the computer to retard timing, costing both power and economy. Follow the manual, not the marketing.
Top-tier detergent gasoline keeps injectors and valves cleaner over 100,000+ miles, which protects MPG long-term. Beyond that, bottled additives rarely move the needle in testing. The honest fuel-economy mods are maintenance, tire pressure, and driving style: everything else is noise.
A Second Worked Example: The Annual Fuel Bill
The per-tank math is easy; the yearly number is the one that changes decisions. Drive 12,000 miles a year in a car that gets 25 mpg. Divide 12,000 by 25 and you burn 480 gallons a year. At $3.50 per gallon, that is $1,680 a year, or $140 a month. Now run the same miles in a 35-mpg car: 12,000 divided by 35 is 342.9 gallons, which costs $1,200 at the same price. The 10-mpg upgrade saves $480 a year, every year you own the car.
Stretch it over a typical 8-year ownership and the gap is $3,840, real money that belongs in the buy-versus-keep math. This is also why small mpg differences matter more on inefficient cars: going from 15 to 20 mpg over 12,000 miles saves 200 gallons, but going from 30 to 35 saves only 57. The gallons, not the mpg, are what you pay for.
MPG and L/100km: The Conversion That Trips Everyone Up
The US uses miles per gallon, most of the world uses liters per 100 kilometers, and the two are reciprocals, not simple multiples. The formula: L/100km equals 235.214 divided by mpg. A 25-mpg car uses 9.4 L/100km; a 35-mpg car uses 6.7. Notice the trap: a 10-mpg improvement from 15 to 25 cuts consumption from 15.7 to 9.4, a 6.3-liter drop, while the same 10-mpg improvement from 35 to 45 only cuts 6.7 to 5.2, a 1.5-liter drop.
This is why European efficiency labels feel nonlinear to American eyes. Each additional mpg buys fewer and fewer gallons saved, because you are dividing by a bigger number. When a carmaker brags about a 2-mpg improvement on a 40-mpg hybrid, do the gallon math before applauding: over 12,000 miles it saves about 14 gallons a year, roughly $50. Honest, but not life-changing.
The 5-MPG Upgrade: What It Really Saves
Dealers love quoting monthly payment differences; run the fuel side yourself. Moving from a 22-mpg SUV to a 27-mpg crossover over 12,000 miles a year: 12,000 divided by 22 is 545.5 gallons; divided by 27 is 444.4. The difference is 101 gallons, worth $354 a year at $3.50. That covers a few car payments' worth of gas money, but it will not single-handedly justify a $8,000 price jump.
The comparison flips for high-mileage drivers. At 25,000 miles a year, the same 5-mpg upgrade saves 211 gallons, or $738 a year, and over 8 years that is $5,900. Mileage multiplies everything. Before paying extra for efficiency, multiply your actual annual miles, not the 12,000-mile fantasy, by the gallon difference. Low-mileage drivers should buy the car they like; high-mileage drivers should buy the efficient one and let the math pay them back.
Pump Math: Octane, Ethanol, and the Price Spread
Premium gas typically costs 50 to 70 cents more per gallon than regular. For a car that merely recommends premium, the math rarely works: 15 gallons times 60 cents is $9 extra per fill-up, about $470 a year for a weekly filler, buying no measurable performance in an engine tuned for regular. For a car that requires premium, usually turbocharged or high-compression, skipping it risks knock and the repair bill dwarfs any savings.
Ethanol blends add a quieter variable. E10, the standard 10-percent blend, carries about 3 percent less energy than pure gasoline, so mpg dips slightly. E85 carries roughly 27 percent less energy, which is why flex-fuel vehicles show a big mpg drop on E85; it only pays when priced at least 27 percent below regular. When the pump shows E85 at $2.80 against regular at $3.50, that is only 20 percent cheaper, and you lose money on energy. Divide price by energy content, not by the gallon, and the bargain often evaporates.
Frequently Asked Questions
How do you calculate fuel consumption?
Divide miles driven by gallons used: 300 miles on 10 gallons is 30 MPG. For metric, divide liters by kilometers and multiply by 100 for L/100km. The calculator above does both and adds trip cost. Measure over full tanks for accuracy rather than partial fills.
What is good fuel consumption for a car?
In the US, 30+ MPG is good for a gas car, 40+ is excellent and usually hybrid territory. Trucks and large SUVs sit around 15-22 MPG. EVs are rated in MPGe, where 100+ MPGe is common. Compare against your own car EPA rating first, then the class average.
How do I calculate fuel cost for a trip?
Divide trip miles by your MPG to get gallons needed, then multiply by the price per gallon. A 400-mile trip at 30 MPG uses 13.3 gallons; at $3.50/gallon that is $46.67. The calculator runs this in one step. Add a 10 percent buffer for detours, idling, and price differences along the route.
Why is my MPG lower than the EPA rating?
Short trips, cold starts, idling, aggressive acceleration, high speeds, and extra weight all cut real-world MPG. The EPA test uses controlled cycles; most drivers see 10-20% less. Winter fuel blends and cold weather alone can cost 10%. Aggressive driving alone can erase 20 to 30 percent versus the test cycle.
Does speed affect fuel consumption?
Strongly. Aerodynamic drag rises with the square of speed, so fuel use per mile climbs fast past 55-60 mph. Driving 75 instead of 65 can raise consumption 15-20%. Cruise control on flat highways helps hold the efficient zone. Every 5 mph over 50 is roughly like paying an extra 20 cents per gallon.
How do you convert MPG to L/100km?
Divide 235.214 by the MPG figure. So 30 MPG is 235.214/30, about 7.8 L/100km. Note the scales invert: higher MPG is better, but lower L/100km is better. The calculator shows both. For example, 25 MPG converts to about 9.4 L/100km.
What hurts gas mileage the most?
Aggressive driving tops the list, wasting up to 30% on the highway. Next come short cold trips, underinflated tires (about 0.2% per PSI low), roof racks and cargo boxes, excess weight, and idling. Fixing driving habits beats any gadget. No additive or bolt-on device has ever beaten a calm right foot in independent testing.
How accurate is the dashboard MPG display?
Usually optimistic by 2-5%. The car estimates fuel flow rather than measuring it precisely. Hand-calculating over several fill-ups, miles divided by gallons at the pump, is the reliable method. Track a few tanks to average out fill variations. The pump and the odometer do not flatter anyone, so trust the hand calculation.