Are compacts better than subcompacts for gas mileage?

It's fairly easy to point at trucks and supercars as the biggest gas-guzzlers on the market, but for the average person on the street it's a lot harder to identify the cars with better gas mileage.

The casual observer might assume that SmartForTwos or Scion iQs are among the most parsimonious vehicles you can buy--after all, they weigh very little and use tiny engines.

As we're aware though, that isn't necessarily the case. Sure, each is economical, but you can buy cars two or three classes above that are equally fuel-sipping.

Stark examples of this "smaller isn't always better" trend can be found in the compact and subcompact segments.

Bigger - Not better, but equal...

Take the Ford Fiesta and Ford Focus SFE-package models, for instance. The Focus is the larger car, with more equipment, greater interior volume and better performance from its 2.0-liter engine.

And yet, despite the Fiesta being smaller, lighter and using a smaller engine, their EPA fuel economy figures are nearly identical--40 mpg highway and 33 mpg combined. The Fiesta is a mere 1 mpg better in city driving, at 29 mpg.

The same is true when comparing hybrids like the Toyota Prius and Prius C. Each shares a 50 mpg combined rating, while there's only a few mpg difference in city and highway driving--the Prius C better at the former, the Prius at the latter. And for interior volume, the Prius is actually a mid-size car.

Of course, the larger cars in each instance are the more expensive to buy, and in that respect it's great for buyers with less cash to spend that they can buy a car with impressive efficiency without spending extra money.

But there is a sense that with the larger vehicles, you're getting better value for money--more space, more equipment and generally, better performance. They may often have larger engines, but particularly in highway driving those larger engines don't need to work as hard.

That's why cars like the aforementioned Smart aren't as frugal as you may expect--its tiny engine has to work overtime to match the pace of larger cars on the freeway, and it's fighting against barn-door aerodynamics as it does so. That's partly why it only gets 38 mpg highway--lower than several subcompacts and compacts.

Which should you buy?

As ever, it's not quite as simple as saying "bigger is better". The car you choose will inevitably depend on the sort of driving you do. If you rarely embark upon highway journeys, then the lightweight car with the smaller engine may return better gas mileage.

And if your budget simply doesn't stretch to a car in the next class up, then the fact it may get equal gas mileage is fairly irrelevant--unless you buy used, of course.

But next time you're in the market for a new vehicle, it may be worth checking out cars in the next class up, just in case you can get the same gas mileage from a bigger car. We're unlikely to see an "upsizing" trend any time soon, but for some buyers, that larger vehicle may actually make sense.