Average mpg for a semi truck
Six to seven miles per gallon, loaded, on the highway. That is the honest answer for a modern Class 8 tractor-trailer, and the spread inside it — 5 to 8 depending on weight, weather, terrain and right foot — is worth more money than most drivers realise.
What each half a gallon is worth
Diesel is $6.53 a gallon — the EIA national average for the week of 21 Sep 2026. At that price:
| MPG | Fuel cost per mile | Per 100,000 miles | Against 6.5 mpg |
|---|---|---|---|
| 5.0 mpg | 131¢ | $130,580 | $30,134 |
| 5.5 mpg | 119¢ | $118,709 | $18,263 |
| 6.0 mpg | 109¢ | $108,817 | $8,371 |
| 6.5 mpg | 100¢ | $100,446 | — |
| 7.0 mpg | 93¢ | $93,271 | $-7,175 |
| 7.5 mpg | 87¢ | $87,053 | $-13,393 |
| 8.0 mpg | 82¢ | $81,612 | $-18,834 |
Half a mile per gallon, from 6.0 to 6.5, is $8,371 a year at 100,000 miles. There is no other lever on the truck that moves that much money for that little effort.
What actually moves the number
| Factor | Rough effect | Notes |
|---|---|---|
| Speed | ≈ 0.1 mpg per 1 mph over 60 | The biggest thing you control. Drag rises with the square of speed. |
| Gross weight | ≈ 0.5 mpg between empty and 80,000 lb | Matters most on hills and in stop-start running. |
| Idling | ≈ 0.8 gal/hour | Does not show as mpg if you log hub miles — it shows as gallons you cannot account for. |
| Tyre pressure | Up to 1% | Cheap, ignored, compounding. Low-rolling-resistance tyres add more. |
| Aerodynamics | 2–8% | Trailer skirts, gap reduction, closed trailer doors when running empty. |
| Terrain and season | Up to 15% | Winter blends carry less energy, and cold engines run rich. Mountains are mountains. |
Why the pump price is not the price you pay
The number above is a national average. Where you fuel changes it as much as how you drive: this week the Gulf Coast average is $6.18, the West Coast is $7.46 and California on its own is $8.25 — a spread of $2.07 a gallon. On a 150-gallon fill that is $310.35.
Planning fuel stops around the regional spread is worth more than another 0.2 mpg, and it costs nothing but attention.
Measuring your own, properly
- Use hub miles and gallons, over a month. Not one tank, and not the dash readout — the dash is optimistic and one tank is noise.
- Count every gallon, including what went in while you were parked.
- Track loaded and empty separately if you can. They are different numbers and knowing the gap tells you what weight is costing you.
- Re-measure after any change. New tyres, a new trailer, a different lane mix — all of it moves the figure.
The calculator takes your real mpg and this week's regional diesel price and works out the fuel on a specific trip, rather than an average of a national average.
Questions
What is the average mpg for a semi truck?
Six to seven miles per gallon for a loaded Class 8 tractor-trailer on the highway. A new aerodynamic truck running light at 62 mph can see 8; the same truck grossing 80,000 lb through mountains in winter can see 5.
What mpg should I use for planning?
6.5 is the standard planning figure and it is what the calculator defaults to. Use your own number if you have it — the difference between 6.5 and your real 6.1 is 7¢ a mile, which is real money over a year.
How much does one mpg change cost?
At $6.53 a gallon, going from 6.0 to 7.0 mpg saves 16¢ a mile — about $15,545 over 100,000 miles. That is the single biggest controllable cost in the operation.
Does speed really matter that much?
Yes, more than almost anything else you control. Aerodynamic drag rises with the square of speed, so the fuel burned overcoming it rises roughly with the cube. The rule of thumb is about 0.1 mpg for every 1 mph above 60 — and running 65 instead of 60 on a 600-mile day saves you around 45 minutes while costing several gallons.
How much fuel does idling use?
Roughly 0.8 of a gallon an hour for a modern engine, more with the air conditioning loaded. Ten hours of idling a night for 200 nights is about 1,600 gallons — the fuel equivalent of 10,000 miles of driving, burned standing still.