How to Calculate Truck Fuel Mileage: MPG, L/100 km and Imperial Gallons

Fuel is usually the biggest bill a truck has, so knowing your real fuel mileage is one of the most useful numbers you can have. For owner-operators it’s profit. For company drivers it’s often a bonus, or just bragging rights at the fuel island.

The tricky part for anyone who runs both sides of the border is the units. Canada talks in litres per 100 km, the US talks in miles per gallon, and older Canadian drivers still sometimes quote imperial miles per gallon. This guide shows you how to calculate each one and convert between them.

Do the math when you’re parked. Never work numbers on your phone while driving.

Step 1: Get honest numbers (the “fill-to-fill” method)

One tank tells you very little. The pump might click off early, or the truck might sit at a different angle. The most reliable method is:

1. Fill the tanks completely and write down the odometer reading.
2. Drive normally. Every time you fuel, write down the litres or gallons you put in.
3. At the end of the period (a week, a month, a few thousand miles), fill completely again and write down the odometer.
4. Distance = last odometer minus first odometer.
5. Fuel used = every fill-up after the first one, including the last one.

The longer the period, the more accurate the result. Track reefer fuel separately if it comes from its own tank, or your truck mileage will look worse than it is.

Step 2: Calculate it

US miles per gallon (mpg):

mpg = miles driven ÷ US gallons used

Litres per 100 km (L/100 km): lower is better.

L/100 km = litres used ÷ kilometres driven × 100

Kilometres per litre (km/L): some drivers like this one because higher is better, just like mpg.

km/L = kilometres driven ÷ litres used

Step 3: Convert between units

These conversion factors come from the US National Institute of Standards and Technology (NIST):

• 1 mile = <strong>1.609344 km</strong> (exact)
• 1 US gallon = <strong>3.785411784 L</strong> (exact)
• 1 imperial (Canadian/UK) gallon = <strong>4.54609 L</strong>

(NIST Handbook 44, Appendix C; NIST SP 811, Appendix B)

That gives you these shortcuts:

• <strong>L/100 km = 235.215 ÷ US mpg.</strong> NIST publishes this exact shortcut (<a href=”https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors&#8221; target=”_blank” rel=”noopener”>NIST SP 811, B.5</a>).
• <strong>US mpg = 235.215 ÷ L/100 km</strong> (the same formula flipped).
• <strong>L/100 km = 282.481 ÷ imperial mpg</strong> (from 100 × 4.54609 ÷ 1.609344).
• <strong>Imperial mpg = US mpg × 1.20095</strong>, because an imperial gallon is about 20% bigger.

That last one trips people up. If a Canadian driver says “7.5 mpg” and a US driver says “6.25 mpg”, they may be describing the same truck.

A worked example

You ran 1,600 km and put in 600 litres after your first fill.

• L/100 km = 600 ÷ 1,600 × 100 = <strong>37.5 L/100 km</strong>
• km/L = 1,600 ÷ 600 = <strong>2.67 km/L</strong>
• US mpg = 235.215 ÷ 37.5 = <strong>6.27 mpg</strong>
• Imperial mpg = 282.481 ÷ 37.5 = <strong>7.53 mpg</strong>

Now the US version: 1,000 miles on 160 US gallons.

• mpg = 1,000 ÷ 160 = <strong>6.25 mpg</strong>
• L/100 km = 235.215 ÷ 6.25 = <strong>37.6 L/100 km</strong>

Turn it into cost per mile or km

Fuel mileage matters because of what it costs you. To get fuel cost per km, take the price per litre and multiply it by L/100 km, then divide by 100. To get fuel cost per mile, divide the price per gallon by your mpg.

For example, at $1.80/L and 37.5 L/100 km, fuel costs $0.675 per km. Plug in your own numbers. Prices change every week, and that’s exactly why it’s worth recalculating.

What moves the number

Natural Resources Canada’s fuel-efficiency guide for highway truckers points to things that are mostly in the driver’s control: steady speed, using cruise control, choosing the right gear, and avoiding unnecessary idling. It notes that, as a rule of thumb, for every 10 km/h above 90 km/h you burn about 10% more fuel (NRCan, Drive Lean, Drive Clean: Highway Trucking). NRCan also offers a free online SmartDriver for Highway Trucking course that takes about 1 to 3 hours (NRCan SmartDriver).

Other things change your mileage too, so compare like with like: loaded vs empty, winter fuel and cold starts, headwinds, mountains vs flat, reefer use, and tire pressure.

Quick checklist

• ☐ Fill to fill, and record odometer and fuel every time
• ☐ Use the same units the whole way through, then convert at the end
• ☐ Don’t mix US and imperial gallons
• ☐ Track reefer fuel separately
• ☐ Recalculate monthly and compare similar runs

Let the calculator do it (free)

The Haul Saints calculators handle fuel mileage in the units you work in, plus travel time and Lima oil conversions. They’re free to use on your phone during a break. Open the calculators

If you’d like a running history of your fuel mileage, along with yearly trip and pay summaries, that’s the kind of thing I’m planning for Haul Saints Pro. Join the waitlist and tell me which features you’d use.

Parked, phone out, numbers in. Then back to driving. Stay safe out there.

Amar, driver since 2018 and founder of Haul Saints

Sources: NIST Handbook 44 Appendix C · NIST SP 811 Appendix B · NRCan Drive Lean, Drive Clean (Highway Trucking) · NRCan SmartDriver for Highway Trucking