I Almost Sold My 15 MPG Truck to Save Gas. The 'MPG Illusion' Showed Me I Was Chasing the Wrong Upgrade
Last month I stood in a dealership trying to decide between two trade-ins for my old pickup, which gets about 15 MPG. Option A was a used crossover rated 25 MPG. Option B was a brand-new hybrid rated 50 MPG. My gut said the hybrid was "twice as good" as the crossover, and that going from 25 to 50 would be the bigger jump in savings. My gut was wrong. Moving from 15 to 25 MPG saves more fuel than moving from 25 to 50 — by a wide margin.
This post is for anyone shopping for a car, comparing a US-rated MPG figure with a European L/100km one, or just wondering why the fuel bill barely moved after an "efficient" upgrade. By the end you'll be able to compare any two vehicles on the number that actually tracks your wallet, and convert between MPG (US), MPG (UK), L/100km and km/L without second-guessing yourself.
⚠️ Disclaimer
This article is for informational purposes only and does not constitute financial advice. Fuel prices, mileage and real-world efficiency vary — always consult a qualified professional for your specific situation.
What Is the MPG Illusion?
The MPG illusion is a measurement trap where miles per gallon makes small improvements to inefficient vehicles look trivial and large improvements to efficient vehicles look huge. MPG measures distance per unit of fuel, but your fuel bill depends on fuel used per distance — the inverse.
Researchers Richard Larrick and Jack Soll described this in a 2008 paper in Science titled "The MPG Illusion." Their finding helped push the US EPA to add a "gallons per 100 miles" figure to new-car fuel economy labels in 2013.
Here's the maths I should have run in that dealership. Assume 10,000 miles a year and gasoline at $3.50 per gallon:
| Upgrade | Gal/yr before | Gal/yr after | Saved/yr |
|---|---|---|---|
| 15 → 25 MPG | 667 | 400 | $933 |
| 25 → 50 MPG | 400 | 200 | $700 |
| 50 → 100 MPG | 200 | 100 | $350 |
Each row doubles or improves by two-thirds the MPG, yet the savings shrink fast. Going from 15 to 25 MPG — a mere 10-point bump — saved me $233 a year more than doubling a 25 MPG car to 50.
Key Takeaway
Fixing the worst vehicle in your household usually saves more fuel than perfecting the best one. Replacing a 15 MPG vehicle with a 25 MPG one saves 267 gallons over 10,000 miles; replacing a 25 MPG vehicle with a 50 MPG one saves only 200.
How Do You Compare Fuel Economy Correctly?
The fix is to flip the ratio. Instead of miles per gallon, look at gallons per 100 miles (or litres per 100 km, which is the same idea in metric). The EPA uses this exact measure on current window stickers because it scales linearly with what you spend.
| MPG (US) | Gal / 100 mi | L / 100 km |
|---|---|---|
| 15 | 6.67 | 15.68 |
| 25 | 4.00 | 9.41 |
| 40 | 2.50 | 5.88 |
| 50 | 2.00 | 4.70 |
Once you have gallons per 100 miles, annual cost is simple:
For my truck: (10,000 ÷ 100) × 6.67 × $3.50 = $2,335 a year. The 25 MPG crossover would cost 100 × 4.00 × $3.50 = $1,400. The hybrid: 100 × 2.00 × $3.50 = $700. The hybrid still wins on fuel alone, but it's $700 better than the crossover, not the "double the savings" my gut imagined — and the hybrid costs far more up front.
💡 Pro Tip
Before you fall for a higher sticker price, divide the extra purchase cost by the yearly fuel saving. A $6,000 premium that saves $700 a year takes about 8.6 years to pay back — before interest. Run the loan side with the auto loan calculator and compare ownership structures with the lease vs buy calculator.
Why Are US MPG, UK MPG and L/100km Different?
Fuel economy ratings come in four common flavours, and the gallon is the culprit. A US gallon is 3.785 litres; a UK (imperial) gallon is 4.546 litres. The same car therefore shows a bigger MPG number in Britain than in America. A car rated at 40 MPG (US) is about 48 MPG (UK) and 5.88 L/100km — identical efficiency, three different numbers.
| Unit | Better when | Used in |
|---|---|---|
| MPG (US) | Higher | United States |
| MPG (UK) | Higher | United Kingdom |
| L/100km | Lower | Europe, Australia, Canada |
| km/L | Higher | Japan, parts of Asia |
The conversion for the two most common systems is a constant:
Here's a UK-flavoured example. Say you drive 12,000 km (about 7,500 miles) a year with petrol at £1.45 a litre (an assumed price — check your local pump). A 15.68 L/100km pickup burns 1,882 litres, or £2,729. A 9.41 L/100km crossover burns 1,129 litres, or £1,637. That's a £1,092 annual difference from a 6-MPG-point jump on the American scale. Australian and Canadian readers can run the same sum with their local price per litre.
Don't want to do the arithmetic by hand? The fuel economy converter translates between all four units instantly and estimates CO₂ per kilometre (petrol burns roughly 2.31 kg of CO₂ per litre).
⚠️ Note
Never compare a US MPG number directly with a UK MPG number. A US-market "30 MPG" car is about 36 MPG on the UK scale. Convert first, then compare.
Why Does Fuel Economy Drop in Winter?
Cold-weather fuel economy loss is the drop in miles per gallon that occurs when temperatures fall, caused by thicker engine and transmission fluids, longer warm-up periods, winter-blend fuel and more cabin heating. The US Department of Energy's fueleconomy.gov reports that conventional gasoline cars can lose roughly 15% of their fuel economy at 20°F compared with 77°F on short trips, and more in very short ones.
That matters this time of year. My 15 MPG truck becomes about 12.75 MPG in a cold spell, which is 7.84 gallons per 100 miles instead of 6.67 — an extra 1.17 gallons for every 100 miles, or about $4.10 at $3.50 a gallon. If you drive 250 miles a week, that is roughly $10 a week of weather tax. Low tyre pressure makes it worse; if the dashboard light is already on, our tire pressure and cold weather guide explains why.
If you're doing a big seasonal drive, see what it costs with the fuel cost calculator. And if you're weighing a plug-in or electric option, the EV charging cost calculator puts the same per-mile thinking on electricity.
Frequently Asked Questions
Is a higher MPG always better?
Yes, higher MPG always means a more efficient vehicle, but the benefit of each extra MPG shrinks as the number rises. The jump from 15 to 20 MPG saves far more fuel than the jump from 45 to 50.
How do I convert MPG to L/100km?
Divide 235.215 by the US MPG figure. For example, 30 MPG (US) is 235.215 ÷ 30 = 7.84 L/100km. For UK MPG, divide 282.481 by the figure instead.
Why does my real-world MPG not match the sticker?
Ratings come from standardised lab tests (EPA in the US, WLTP in Europe). Speed, temperature, tyre pressure, cargo, short trips and aggressive driving all push real-world numbers lower, often by 10–20%.
Is lower L/100km better or worse?
Lower is better. L/100km measures fuel used per distance, so a 5 L/100km car uses less fuel than an 8 L/100km one — the opposite direction from MPG.
Do electric cars use MPG too?
The EPA uses MPGe (miles per gallon-equivalent), where 33.7 kWh of electricity is treated as one gallon of gasoline. For budgeting, cost per mile or kWh per 100 miles is more useful than MPGe.
Try It Yourself
The cheapest fuel in the world is the gallon you never burn, and the MPG illusion hides where those gallons are. Compare vehicles on gallons per 100 miles (or L/100km), multiply by your real yearly mileage, and let the numbers decide.
Start with the Fuel Economy Converter to translate any rating into the unit you trust, then price your real routes with the Fuel Cost Calculator. If a new car is on the table, check the payments with the Auto Loan Calculator.
