When is a Tundra a better buy than a Prius?

This never fails to fascinate me:

The chart shows how much fuel is consumed over 15,000 miles by cars of different fuel efficiencies.

The curve matters a lot. It means that from the perspective of fuel conservation, it’s not terribly important to trade in your Honda Civic to buy a Prius. But it’s hugely important to trade in your Dodge Durango for a Toyota Tacoma.

I’ll use some rough numbers to illustrate. You trade in your Civic, which averages about 32 miles per gallon, and buy a Prius, which gets a whopping 47 mpg. You’ve bumped up by 15 mpg — a big deal, right?

Sort of. Over the next 15,000 miles of driving, you’ll have reduced your fuel consumption by 150 gallons. That’s fine. But consider what happens when you upgrade your SUV. That’s where the real action is.

You swap out your Dodge Durango (16 mpg on average) for a Toyota Tacoma (23 mpg). It’s an upgrade of just 7 miles per gallon. It seems tiny. But consider that over the next 15,000 miles, you will have saved 285 gallons of fuel — nearly double what your fuel-sipping neighbor saved.

It’s a mind-bender, I know. But that’s math for you. And that’s what the chart illustrates. If we want to maximize fuel conservation, we need to concentrate on places where we can move quickly down the steep part of the curve. Once we’ve gotten down to the corner — around 25 or 30 miles per gallon — we won’t get nearly the payoff from efficiency improvements.

Incidentally, this matters a lot for climate policy too. Each gallon of fuel burned translates directly into about 20 pounds of carbon dioxide in the atmosophere. So the curve applies equally to fuel economy and global warming.

Okay, enough pedantry. Here are the take-away lessons:

1. To reduce fuel use, our public policies should focus on small upgrades to the least efficient vehicles. It’s less important to tinker at the upper end. The biggest gains are at the low end — and small improvements make an enormous difference.
2. The U.S. should take a cue from Canada. We should talk about “gallons per mile,” not “miles per gallon.” (North of the border, of course, it’s litres per kilometre.) “Gallons per mile” makes it much, much easier to see where the problem lies — at the low end.

My boss, Clark Williams-Derry, has looked at this phenomenon before. You can find his insights here, here, and most importantly, here.