Despite best intentions, it's always necessary to take EPA fuel efficiency figures with a pinch of salt.
And that, of course, is why the slogan "Your mileage may vary" came into being.
People have known this for as long as official tests have been carried out. Certainly since the old VW bug, if the advert you see above is to be believed.
Volkswagen has long since been renowned for its self-deprecating advertising style, often implying that its products might not be the best (particularly during the Beetle era), but selling their virtues with brilliant effect.
And while the Beetle might have achieved impressive fuel economy figures in official testing, VW was more keen to promote the actual figures that real-world driving might result in--in this case, an "honest 25 mpg".
In completely non real-world driving, with a significantly lightened body--and even a jockey behind the wheel to reduce weight, at least for amusement purposes--the Beetle would be capable of up to 84 mpg.
In more recent years, Volkswagen has been on the other side of gas mileage claims. Many drivers in the real world are finding their Volkswagen diesels will achieve comfortably more than that seen in EPA testing--though no carmaker these days would put its neck on the line and suggest their cars beat official figures... and those that can't even match them are at risk of getting sued.
Gas mileage depends on far more than just the type of car you drive. Regularly encounter lots of heavy, stop-and-go traffic? Expect to see less than even the official city figure (unless you drive a hybrid...). Drive 90 mph on the freeway? Don't expect to match that 40 mpg figure. Regularly do 80 miles a day in your Chevy Volt without a top-up charge in the middle? That combined economy figure will seem harder to hit.
Then there's weather, driving style, and landscape. One-way journeys can look pretty bad on your trip-computer if you've been going up-hill a lot (just as well you'll be on an easy downhill glide in the other direction...).
There's maintenance too--keep those tires pumped up and in good condition, and you'll have your best chance of getting good mileage. And accessory usage--keep aircon and lights to a minimum (safety permitting) and your wallet will benefit. Do the complete opposite, and don't expect to match those window stickers.
So while you're shopping for your next car, take a look around some of the real-world economy websites, and see what other drivers have been getting and on what sort of journeys.
You might just save some gas by buying a model that beats its EPA ratings--or at least, reduce the surprise of a model that doesn't...