2010 Toyota PriusEnlarge Photo
Toyota knew this when it developed the 2010 Prius.
Compared with 2004-2009 models, it used a bigger engine--1.8 liters versus 1.5. Yet MPG still went up, and users on Fuelly are achieving a few mpg more despite having more performance.
While smaller engines should use less fuel (all other things being equal) many seem to struggle doing so when they also have to haul along a relatively heavy modern car body.
And they struggle even more in cut-and-thrust traffic, when the driver needs a little more performance. This is why many subcompacts achieve little better mileage than larger compact cars.
So how small is too small, then?
It clearly depends on the application, for a start. While a 900cc turbocharged engine in a Fiat 500 is no better than a 1.2-liter four-pot available in Europe, the 1.2 is recording figures almost 7 mpg better than the slightly larger 1.4-liter unit--so 1.2 liters is probably the sweet spot for the 500.
Likewise, the Toyota Prius may not necessarily be made more efficient by increasing the engine to a 2.0-liter unit. And buyers are over-achieving when it comes to economy in the 1.4-liter, turbocharged Chevrolet Cruze Eco.
But significantly downsizing to chase impressive EPA figures may not be as beneficial as it sounds.
Not only do many of these vehicles seem unable to match their figures in the real world, but the complicated, high-tech engines often cost a little more in the first place.
When the 2014 Ford Fiesta hits U.S. shores with the 1.0 EcoBoost engine and a headline efficiency figure, we'd advise you to treat it with suspicion--particularly given recent concerns over Fords missing their official EPA figures.
Of course, in the smaller, lighter Fiesta, the EcoBoost may be fine--which is really our point.
Some engines are better suited to some cars than others--buy too small, and "your mileage may vary" just a little more than you were expecting...