How to Make Things Faster

Chapter 84. The MPG Problem

The hit rate fallacy is a problem with ratios in general. For this story, let’s use an automotive metric with which you’re probably already intimate: miles per gallon (mpg). The mpg metric is defined simply as the number of miles you drive divided by the number of gallons of fuel that you burn. It’s an efficiency metric. More efficiency is always good, so it makes sense that mpg is a higher-is-better metric. But, as innocent as a simple “output divided by input” ratio sounds, even efficiency itself is indeed susceptible to ratio fallacies.

Imagine that there are two distinct routes to your office at work: call them A and B. When you take route A, your fuel efficiency is 20 mpg. On route B, your mileage is 30 mpg. Which route should you choose? It seems an easy choice: more efficiency is better, and 30 mpg is more than 20 mpg, so then 30 mpg is better. Therefore, obviously, route B is better. It’s just common sense.

Except, well…it’s the wrong answer. Let’s look at what’s really going on:

Measure	Route A	Route B
Distance	5 miles	20 miles
Duration	10 minutes	25 minutes
Fuel burned	0.25 gallons	0.67 gallons
Notes	Stop-and-go city driving	Steady freeway driving
Fuel efficiency	20 mpg	30 mpg

So, which route should you take? It’s obvious when you look at more facts: route A is 15 miles shorter, 15 minutes faster, and uses nearly half a gallon less fuel. Route A is the superior route.

That’s right: route A, with the worse mpg figure, is better. ...