Which saves more fuel: replacing a 12 MPG truck with a 14 MPG truck, or replacing a 30 MPG car with a 40 MPG car? Almost everyone says the car swap. The correct answer is the truck: going from 12 to 14 MPG saves 119 gallons per 10,000 miles, while going from 30 to 40 MPG saves only 83 gallons. The MPG scale is a frame that systematically misleads.
The Framework
The MPG illusion demonstrates that miles-per-gallon is a nonlinear scale that produces framing effects. The relationship between MPG and fuel consumption is a curve, not a line: improvements at the low end of the MPG scale save far more fuel than equivalent improvements at the high end. But the MPG frame presents both improvements as numerically equal (2 MPG vs. 10 MPG), making the larger actual savings look smaller.
The fix is simple: replace MPG with gallons-per-mile (or its equivalent, liters per 100km, which Europe and most of the world already uses). On a linear scale, the comparison is transparent. The MPG illusion is a pure framing effect — the same information, presented differently, produces different decisions.
Where It Comes From
Larrick and Soll published the MPG illusion research in 2008. Kahneman presents it in Chapter 34 of Thinking, Fast and Slow as an example of how framing affects even quantitative, factual decisions. The policy implication: requiring fuel economy to be displayed as gallons-per-mile would shift consumer behavior toward replacing the least efficient vehicles first — where the fuel savings are largest.
> "The message of these examples is simple: unless there is an obvious reason to do otherwise, you should favor the linear scale." — Thinking, Fast and Slow, Ch 34
Cross-Library Connections
Dib's pricing transparency advice in Lean Marketing aligns with the MPG lesson: present information on scales that allow honest comparison. Nonlinear scales (percentage discounts, monthly-vs.-annual pricing) create framing effects that may mislead customers.
The Implementation Playbook
Data Presentation: Always check whether your metrics use linear or nonlinear scales. Conversion rates, percentage improvements, and ratio metrics can all produce MPG-like illusions. When comparing options, convert to a linear scale (absolute units) before deciding.
Pricing: "Save 50% off a $20 item" ($10 savings) sounds better than "Save 10% off a $200 item" ($20 savings) because percentage is a nonlinear frame. Use absolute savings when the comparison favors your product; use percentage savings when the percentage is more impressive. Be aware you're choosing a frame.
Environmental Policy: Display vehicle efficiency in gallons-per-mile, not MPG. Display energy efficiency in kWh-per-year, not efficiency ratings. Linear scales produce better consumer decisions because they allow honest comparison.
Business Metrics: When comparing two initiatives, convert improvement percentages to absolute values. A 10% improvement in a $10M division ($1M) may be worth less than a 2% improvement in a $100M division ($2M) — but the percentage frame makes the 10% look more impressive.
Key Takeaway
The MPG illusion proves that even simple, factual, quantitative decisions are susceptible to framing effects. The scale you present information on — linear vs. nonlinear, absolute vs. relative, per-unit vs. total — determines the decision. Always check: is the scale I'm using producing an honest comparison?
Continue Exploring
[[Framing Effects]] — The broader principle of which the MPG illusion is a quantitative example
[[Evaluability Hypothesis]] — When attributes are hard to evaluate in isolation, the scale determines the judgment
📚 From Thinking, Fast and Slow by Daniel Kahneman — Get the book