If you want the best fuel economy, don't buy the smallest cars. Yes, that is right.
Drivers looking for savings at the petrol pump could be making a mistake if they swap their estate or 4x4 for a smaller car, according to research which suggests that fuel economy estimates are biased against larger vehicles. . . .
The discrepancy between manufacturers’ claims and the road data was especially stark for vehicles with smaller engines, which generally have to work harder to accelerate.
Tests showed that vehicles with an engine size up to one litre had an average advertised 60.3mpg, but consumption was measured at 38.6mpg in tests, a drop of 36 per cent.
Average consumption for cars with one to two-litre engines was measured at 46.7mpg, 21 per cent lower than the advertised 59.1mpg. This meant they travelled further on the same amount of fuel than the average smaller car. . . .It is also interesting that there is almost no difference in fuel economy between midsize and larger cars.
There are at least a couple US cars with 1 Liter engines.
2014 Ford Fiesta 1.0L EcoBoost
2014 Smart fortwo
UPDATE: Here is something that I found at Green Car Reports:
The worst offenders were vehicles with engines displacing 1.0 liter or less (no sub-1.0-liter engines are available in the U.S.), which missed the mark by 36 percent. Engines between 1.0 liter and 2.0 liters were second worst, at 21 percent. . . .