Semi truck MPG calculator

How the calculator works

Fuel economy is miles divided by gallons, but a single tank tells you little. Fill-ups vary, pumps shut off early, and one hilly run can skew the number. Logging several fill-ups gives a truer average.

1. Each fill-up. MPG = miles since the last fill-up / gallons to fill back up.
2. Average. All miles logged / all gallons logged. This weights long runs properly, unlike averaging the MPG figures.
3. Fuel cost per mile. Diesel price / average MPG.
4. Loaded vs light. Fill-ups with 30,000 lb or more of cargo are grouped as heavy, the rest as light or empty, and each group gets its own MPG.

The bar chart shows each fill-up's MPG side by side, colored by load weight, so a drop stands out quickly.

A worked example

A driver logs four fill-ups at $3.90 a gallon: 1,080 miles on 162 gallons with 38,000 lb, 1,120 miles on 160 gallons with 41,000 lb, 940 miles on 131 gallons with 12,000 lb, and 1,010 miles on 158 gallons with 44,000 lb.

Reading your result

Your average MPG and fuel cost per mile feed straight into the rest of your numbers. Use the cost per mile in your trucking cost per mile calculation, and use the average MPG in the trip fuel cost calculator when you price a load.

Watch the trend more than any one fill-up. A single low number can come from a mountain pass, a headwind or a long idle. A steady slide over several fill-ups on similar loads usually means something has changed: tire pressure, a dragging brake, an injector, an aftertreatment problem or speed creeping up.

Getting accurate numbers

• Fill to full each time. The gallons that fill the tanks back up are the gallons burned since the last fill. Partial fills break that link.
• Use the same pump habit. Let the nozzle click off, then stop. Topping off differently each time adds noise.
• Log every fill. Skipping one fill-up means the next one covers two stretches. Add them together if that happens.
• Note reefer and idle use. If a reefer or heater draws from the main tanks, your MPG will look lower than the engine's real economy.

When a drop means a problem

A sudden fall in MPG on a familiar lane with a normal load is worth checking before it turns into a breakdown. Start with the cheap things: tire pressure on every wheel, a brake that drags, and the trailer's alignment. Then look at the engine side: air filter, injector issues, a clogged or regenerating aftertreatment system and fault codes. A shop visit that finds a problem early usually costs less than the fuel it was wasting and far less than a roadside repair.

Keep the log going after a repair. If MPG comes back to where it was, you've confirmed the fix; if it doesn't, the cause is still there.

Speed, gears and fuel

Speed is the biggest factor most drivers control. Wind resistance climbs steeply with speed, so a few miles per hour can show up clearly on the pump receipts. The speed limiter impact calculator shows the trade between time and fuel over a week. Gearing matters too: running in the engine's efficient range saves fuel. Our guide on how many gears a semi truck has explains how modern transmissions handle that for you.

Curious about trucks that skip diesel altogether? Our guide to the electric semi truck looks at range, charging and where battery trucks fit today.

Fuel and empty miles

MPG measures how well the truck uses fuel. Empty miles measure how much of that fuel earns money. That second number is the one a dispatcher can move. Ours look for the reload while you're still loaded, which trims the empty miles your MPG can't fix. The owner-operator page shows a week planned that way, and the trip fuel calculator prices a single run.