How The EPA Figures MPG – And Why It Doesn’t Work

Last Fall Consumer Reports made headlines regarding their testing of two new Ford hybrids:  the C-MAX and the Fusion.  At the time both had EPA mileage stickers claiming the vehicles average 47 miles per gallon in combined city and highway driving.  However, according to Consumer Reports, the Fusion averaged only 39 miles per gallon, the C-MAX 37.  Consumer Reports blames the way Ford hybrids work.  A better report would have blamed the EPA.

Many people are under the impression the EPA tests every automobile over a predetermined driving route; one that consists of some city driving and some highway driving.  Then, at the end of the test, they measure the amount of fuel used and report the mileage per gallon.  Nothing could be further from the truth.   The fact is the EPA does very little testing; the mileage estimates are provided by the manufacturers.  The EPA then selects 10 to 15% of the vehicles at random and retests them to make sure the mileage estimates provided by the manufacturers are accurate.

When the EPA does test a car this is how they do it.  They put the car on an dynamometer inside a laboratory;  the dynamometer has a set of rollers which the vehicle rests on while being tested.  By varying the rollers resistance to rotation the dynamometer simulates the effect of stop-and-go driving, highway driving, driving up a steep hill and even driving into a headwind.  Many believe this has about as much relevance to real world driving as riding an exercise bike has to cycling.

car_on_dynoThe test procedures used by Consumer Reports are dramatically different than those used by the EPA.  Consumer Reports buys cars from dealerships and then drives them on a preset route.  However, because of their design Consumer Reports testing tends to penalize hybrids like the Fusion or the C-MAX.  Ford appears to have optimized the C-MAX and Fusion for city driving; both vehicles only go up to 62 miles per hour in pure electric mode.  The Ford hybrid setup works well for EPA testing, where a vehicle averages 48 miles per hour, but not with the Consumer Reports highway test.  For the highway portion of their testing Consumer drives their cars at a constant speed of 65 miles per hour.

Consumer Reports obviously has a more realistic test for highway driving compared to the EPA’s which tends to penalize hybrids.  An additional problem is how Consumer Reports determines a vehicle’s combined city and highway mileage.  They assume you spend 62% of your their time highway driving and only 38% in city driving.  This weights Consumer Reports averages towards cars that perform well on the highway.  Unlike Consumer Reports, EPA testing assumes you average 55% of their time in city driving and the rest is highway driving.  This is one instance where I suspect the EPA got it right.

Based on Consumer Reports testing the Ford hybrids average about 20% lower than their EPA estimates, the worst of any car recently tested, but what about other hybrids?  The fact is, Consumer Reports own test data shows the EPA mileage estimates for other hybrids is just about as bad as they are for Ford’s hybrids.  The EPA estimate for the Toyota Prius v is 44 miles per gallon in the city, but Consumer Reports says the Prius v gets just 33 miles per gallon in the city.  Consumer Reports admits in their article the only reason the Prius models even come close to the EPA’s combined mileage estimates is because their highway MPG is so good.

The reality is hardly any car matches it’s EPA numbers, and hybrids have an especially hard time of it.  Ford’s hybrids are no better, nor no worse, than any other manufacturers.  Like the other manufacturers, Ford follows the testing procedures established by the EPA.  Because the C-MAX and Fusion hybrids appear to be designed with city driving in mind, they didn’t do very well in the mileage tests conducted by Consumer Reports.  Consumer Reports then compounds the problem by weighting their city and highway mileage estimates differently than the EPA.  It seems to me that both Ford and the EPA need to do a better job when it comes to estimating miles per gallon.