MPGs, meet GPMs

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If you’re looking to save gas, upgrading from a 28 mpg Ford Focus to a 46 mpg Toyota Prius is certainly a better choice than upgrading from a 15 mpg Dodge Durango to a 20 mpg Nissan Murano, right? Wrong. Over 10,000 miles, choosing the Murano-Dodge swap will save you 26 more gallons than the Prius-Focus switch.

The detailed answer as to why the Durango-to-Murano switch is a bigger gas saver puts this forum right at the heart of a fuel-efficiency labeling polemic that, if not at full, raging debate status yet, perhaps ought to be.

As any red blooded American will tell you, fuel economy is measured in miles per gallon (mpg). Unless, of course, you’re in Europe, where it’s measured in liters per 100 kilometers (l/100km).

To avoid any conversion migraines, not to mention the usual disdain, teeth-gnashing and downright fury provoked by all things metric, think of the European standards as gallons per mile (GPM). That’s how Duke University business school professors Richard P. Larrick and Jack B. Soll did it in a fascinating recent study of the subject, called ‘The MPG Illusion’ in the respected journal Science.

They examined how consumers fail to do the math required to figure out how much gas a particular vehicle will consume, focusing instead on the MPG number alone. The problem is that improvements in MPG have an inverse relationship to fuel savings, which is a fancy way of saying that fuel savings are dramatically reduced as you move further up the fuel economy scale.

Unfortunately, that fact escapes all but the most mathematically-inclined observers. The Toyota Prius in the above example consumes 217 gallons to go 10,000 miles, while the Focus consumes 357 -- an 18 mpg difference, or 64%, that yields 140 gallons in savings. The Murano, on the other hand, consumers 500 gallons, compared to 666 for the Durango -- a 5 mpg, or 33%, difference that yields 166 gallons in savings

But if vehicles were measured in GPMs -- expressed, say, as gallons per 1,000 miles -- the actual fuel savings between any two vehicles would be far more obvious. The Prius rates a 21, the Focus a 35.7, the Murano a 50 and the Durango a 67, and it’s immediately clear that the latter switch is better than the former.

This reasoning can be helpful, for example, for families considering whether to ditch the gas guzzler or the midsized sedan in their quest to be more fuel efficient. As the authors of the study say

Perhaps the most famous (infamous?) application of this reasoning was performed by Green Car Journal, which, in a surprise decision, named the Chevrolet Tahoe Hybrid its green car of the year for 2008. The behemoth hybrid gets only 21 mpg, compared to 16 mpg for the non-hybrid version. That provoked scorn from environmentally conscious drivers who felt unimpressed by a 21 mpg rating. But using GPM, it’s suddenly clear that over 10,000 miles, the hybrid saves a driver an impressive 149 gallons, or 24%.

Of course, there is a big canard buried in these kinds of comparisons: they make the assumption that the only alternative to something like the Chevy Tahoe is the Chevy Tahoe Hybrid. If the driver instead chooses a Toyota Highlander hybrid, rated at 26 mpg, the savings work out to 240 gallons, or 39%. And if they pick a Prius...

-- Ken Bensinger