A miles per gallon calculator is a tool that divides the miles you drove by the gallons of fuel you used, giving you MPG, km/L, L/100 km, and fuel cost per mile in a single step.
Key Takeaways
- U.S. average diesel hit $5.65 a gallon the week of August 24, 2026, a new high for the year, according to EIA weekly diesel data reported by Overdrive.
- The average U.S. truck fleet gets about 7 MPG, up from 6 MPG fifteen years ago, while the top-performing diesel trucks in NACFE’s March 2026 Run on Less test hit 11.5 MPG.
- Fuel now averages $0.48 per mile, about 20.5% of the $2.34 total per-mile cost of running a truck, per ATRI’s 2026 operational cost benchmarking.
- An idling truck burns roughly 1 gallon of diesel per hour while covering zero miles, according to Argonne National Laboratory.
- Keeping tires properly inflated recovers 1.4% to 1.8% in fuel economy fleet-wide, based on FMCSA’s Tire Pressure Monitoring and Maintenance Systems Operational Test.
- Driving 70 mph instead of 60 mph cuts fuel economy by about 14%, according to the U.S. Department of Energy.
- This calculator converts between MPG, km/L, and L/100 km automatically, so mixed units like miles with liters never break the math.
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How does a miles per gallon calculator work?
Miles per gallon equals the distance you drove divided by the fuel you used to drive it: MPG = miles driven ÷ gallons consumed. Fill the tank, zero your trip odometer (or note the mileage), drive until your next fill-up, then divide the miles on the odometer by the gallons it took to top off the tank again. The calculator below does that math for you, along with the metric equivalents and your cost per mile if you enter a fuel price.
Miles Per Gallon Calculator
Works in miles or kilometers, gallons or liters. Mixed units convert automatically. Fuel price is optional.
What is a good MPG for different types of vehicles?
A “good” MPG depends entirely on the vehicle class. A compact car and a loaded Class 8 semi truck are not competing on the same scale, so compare your numbers against your own vehicle type rather than a generic target.
| Vehicle type | Typical MPG | Source |
|---|---|---|
| Compact or midsize car | 30 – 40 MPG | FuelEconomy.gov |
| Half-ton pickup truck | 18 – 24 MPG | EPA combined ratings, 2026 model year |
| Box truck / straight truck | 4.0 – 10.1 MPG by class | U.S. Department of Energy |
| Class A gas motorhome | 6 – 10 MPG | RV industry reporting |
| Class A diesel pusher motorhome | 7 – 12 MPG | RV industry reporting |
| Class 8 semi truck, national fleet average | 7 MPG | NACFE Run on Less, March 2026 |
| Class 8 semi truck, top performers | 9.5 – 11.5 MPG | NACFE Run on Less, March 2026 |
Why does diesel cost so much more in 2026, and why does MPG matter more now?
The national average price of on-highway diesel reached $5.65 a gallon the week of August 24, 2026, a new high for the year and roughly $0.20 higher than the week before, according to EIA data reported by Overdrive. California drivers paid $7.04 a gallon, while the Lower Atlantic region paid the least at $5.35. That is well above the prior 2026 peak of $5.64 set back in March and April.
Every 1 MPG improvement matters more when fuel costs this much. A fleet running 10,000 miles a month at 6.5 MPG instead of 7 MPG burns roughly 110 more gallons a month, which is about $620 extra at $5.65 a gallon. Run that across a dozen trucks for a year and the gap becomes six figures. See our breakdown of the story behind rising trucking costs for the full picture beyond fuel, or work through calculating the real cost of trucking per mile to see where fuel ranks against every other expense.
Fuel is not a minor line item. ATRI’s 2026 operational cost benchmarking puts the average total cost of running a truck at $2.34 per mile, with fuel at $0.48 per mile, or about 20.5% of the total, according to FleetOwner’s coverage of the ATRI report. Only driver pay and truck/trailer payments typically outrank it.
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What is the difference between MPG, km/L, and L/100 km?
All three describe fuel efficiency, just measured differently. MPG (miles per gallon) tells you how far a vehicle travels on one gallon of fuel, and a higher number means better efficiency. Km/L (kilometers per liter) is the metric equivalent, again with a higher number meaning better efficiency. L/100 km (liters per 100 kilometers) flips the logic: it tells you how much fuel is needed to cover 100 kilometers, so a lower number means better efficiency. This calculator returns all three at once so you never have to convert by hand or remember which direction is “better.”
What factors change your fuel economy from one fill-up to the next?
A single fill-up rarely tells the full story. Fuel economy shifts fill-up to fill-up because of a handful of variables that compound on each other:
- Speed. Aerodynamic drag increases sharply above 60 mph. The Department of Energy found that fuel economy drops about 14% when driving 70 mph instead of 60 mph, and each 5 mph you drive over 50 mph acts like paying roughly $0.21 more per gallon, according to FuelEconomy.gov.
- Idling. A heavy truck idling burns about 1 gallon of diesel per hour while covering zero miles, per Argonne National Laboratory research for the Department of Energy.
- Tire pressure. Underinflated tires increase rolling resistance. FMCSA’s Tire Pressure Monitoring and Maintenance Systems Operational Test found fleet-wide fuel economy gains of 1.4% to 1.8% from proper inflation alone. See why steer tire inflation matters for the full maintenance angle.
- Cargo weight and load balance. Every extra pound the engine has to move costs fuel, and an unevenly loaded trailer increases rolling resistance further.
- Weather and terrain. Headwinds, cold-start fuel enrichment, and grade all reduce MPG measurably compared to flat, calm-weather driving.
- Aerodynamics. Trailer skirts, gap reducers, and boat tails cut drag on Class 8 combinations and are a large part of how the top NACFE performers reached 11.5 MPG.
- Driver behavior. Hard acceleration and braking waste fuel that steady, anticipatory driving does not.
Because any single fill-up mixes these variables together, track MPG across several fill-ups rather than judging your vehicle or fleet off one tank. A telematics platform can automate this tracking at fleet scale; see how telematics is revolutionizing trucking for what that looks like in practice.
How can you improve your truck’s MPG?
The five changes above the fold of this section are the ones with the most sourced evidence behind them, in order of how quickly they pay off:
1. Track every fill-up
You cannot improve what you do not measure. Log distance, gallons, and price at every fill-up (this calculator or our diesel fuel calculator make that fast) so trends show up before a bigger problem does, like a failing injector or a slow tire leak.
2. Check tire pressure weekly
A 1.4% to 1.8% fleet-wide gain from proper inflation is one of the cheapest wins available. Cold-check pressure, not right after driving, since heat inflates readings temporarily. See why proper tire inflation matters for a technician’s-eye view.
3. Cut unnecessary idling
At roughly 1 gallon burned per hour for zero miles driven, idling is pure loss. Auxiliary power units, bunk heaters, and stop-start policies all pay for themselves faster than most fleets expect.
4. Slow down on the highway
Dropping from 70 mph to 65 mph recovers a meaningful share of that 14% gap without adding meaningful time to most runs, especially over long-haul distances where the math favors steady, moderate speed.
5. Add aerodynamic trailer devices
Trailer skirts, tails, and gap reducers are a large part of how NACFE’s top-performing trucks separated themselves from the 7 MPG fleet average. They cost money upfront but keep paying every mile after that, and they pair well with the broader push toward smart trailer technology fleets are adopting in 2026.
How much does 1 MPG actually save a fleet per year?
Run the math on a single truck at 100,000 miles a year. At 6 MPG, that truck burns 16,667 gallons. At 7 MPG, it burns 14,286 gallons, a difference of 2,381 gallons. At $5.65 a gallon, that 1 MPG improvement is worth about $13,450 per truck per year. Multiply that across a 10-truck fleet and it is roughly $134,500 annually, which is why fuel efficiency gets treated as a fleet-management priority and not just a driver-behavior footnote.
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What is the difference between MPG, km/L, and L/100 km?
MPG and km/L both measure distance per unit of fuel, so a higher number is more efficient. L/100 km measures fuel needed per fixed distance, so a lower number is more efficient. This calculator converts between all three automatically.
Can I mix units, like entering miles with liters?
Yes. Select miles or kilometers for distance and gallons or liters for fuel independently. The calculator converts everything behind the scenes before showing results, so mismatched units never produce a wrong answer.
Why does my fuel economy change from one fill-up to the next?
Speed, idling time, tire pressure, cargo weight, weather, terrain, and driving style all affect a single tank’s result. Track several fill-ups over time rather than judging efficiency off one number, since short-term swings are normal.
What is a good MPG for a semi truck?
The national fleet average is about 7 MPG as of NACFE’s March 2026 testing, up from 6 MPG fifteen years ago. Top-performing Class 8 trucks with aerodynamic packages and disciplined driving hit 9.5 to 11.5 MPG in the same test.
How much does 1 MPG improvement save a trucking fleet per year?
On a single truck running 100,000 miles a year, going from 6 MPG to 7 MPG saves about 2,381 gallons annually, worth roughly $13,450 at $5.65 a gallon. Across a 10-truck fleet, that is about $134,500 a year.
Why did diesel prices spike in 2026?
National average on-highway diesel hit $5.65 a gallon the week of August 24, 2026, a new high for the year, according to EIA data. Regional prices vary widely, from $5.35 in the Lower Atlantic to $7.04 in California.
Does idling actually hurt fuel economy that much?
Yes. A heavy truck idling burns roughly 1 gallon per hour while covering zero miles, according to Argonne National Laboratory. Over a year of unnecessary idling, that adds up to real money with nothing to show for it in miles driven.
How often should I check tire pressure to protect fuel economy?
Check cold pressure weekly at minimum, and before any long haul. FMCSA field testing found tire pressure monitoring and maintenance systems improved fleet-wide fuel economy by 1.4% to 1.8%, one of the least expensive gains available to a fleet.