Comparing the official fuel efficiency figures for several cars, I was surprised to discover that the Smart Fortwo has a weight between 750 to 800 kg and needs 4.3 L/100 km, but the VW Polo with a weight of 1650 kg reaches 3.8 L/100 km. That is, the Smart has half the weight, but still a worse fuel efficiency.
When driving up and downhill (but not on a flat surface) does one cancel the other? That is, driving uphill with more weight costs more fuel, but you need less fuel when going downhill.
Are the major fuel efficiency improvements rather on other places, like better tires, better injection, better streamlining and hence lower weight cannot outweigh poor performance in these fields?