The MPG Illusion: Why MPG Isn’t Linear (and What to Use Instead)
- Going from 10 to 20 MPG saves far more fuel than going from 30 to 40 MPG (over the same miles).
- Use gallons/100 miles or cost/100 miles for clearer comparisons.
- EPA label fuel consumption is designed to connect efficiency to real spending.
Why MPG Isn’t Linear
MPG is “miles per gallon.” If you double MPG, you don’t always double your savings—because fuel savings depend on how many gallons you burn for a given distance.
For any fixed distance:
- Fuel used = Distance ÷ MPG
- Fuel saved = (Distance ÷ MPGold) − (Distance ÷ MPGnew)
That “distance divided by MPG” relationship is why small changes at low MPG matter a lot, and the same change at high MPG can matter less.
Examples: Same MPG Gain, Different Savings
Here’s a simple comparison over 10,000 miles (math only; fuel price doesn’t matter yet):
| Scenario | Old MPG | New MPG | Gallons used (old) | Gallons used (new) | Gallons saved |
|---|---|---|---|---|---|
| Low-MPG improvement | 10 | 20 | 1,000 | 500 | 500 |
| Higher-MPG improvement | 30 | 40 | 333 | 250 | 83 |
Both examples are “+10 MPG,” but the fuel saved is very different. That’s the core of the MPG illusion.
What to Use Instead (EPA Fuel Consumption)
EPA’s label materials include a Fuel Consumption Rate in gallons per 100 miles. EPA explains in the interactive gasoline label that consumption relates directly to fuel used and fuel expenditures.
We break down gallons/100 miles (and conversions) here: Gallons per 100 miles explained.
If you want dollars, convert consumption to a cost metric:
How to Apply This When Shopping
When you compare vehicles:
- Use Combined MPG for a quick scan.
- Use gallons/100 miles to understand fuel use differences.
- Use your own miles and local prices to estimate annual cost.
EPA explains label cost estimates and assumptions in the fine print section of the label (see the interactive label details). You can personalize that quickly using our Commute Gas Cost Calculator or Fuel Cost Calculator.
Turn MPG Into Dollars
Enter your MPG, miles, and gas price to estimate fuel cost per mile and annual fuel spending.
Use the Cost Per Mile CalculatorFrequently Asked Questions
Is MPG useless, then?
No. MPG is helpful for quick comparisons. The point is that MPG gains don’t translate evenly into fuel savings. If you’re deciding between vehicles, add gallons/100 miles (fuel consumption) to your comparison.
Where can I find gallons per 100 miles?
EPA includes it as a “Fuel Consumption Rate” on the gasoline vehicle label, explained in the interactive label.
Is this the same idea as L/100km?
Yes. Both are “fuel used per fixed distance,” just in different units. See our metric conversion guide: MPG to L/100km.
How does this help with driving tips?
It helps you focus on changes that actually reduce fuel used. For example, you can estimate gallons saved per month from a change in MPG using your miles driven and fuel price, then decide whether the change is worth it.