[oldtamiyaphile]

Why in the world would you assume that a 20 mph average means 20mph speed limits? 20mph average means lots of stop and go (average 1 traffic light per km). Weight should kill my mpg but it doesn't, I simply drive around the fact that I've got a 1.6l engine out of a hatchback pulling a train.

Drive any (ICE) vehicle at 20mph (3rd gear in my tall gearbox) and you get abysmal fuel economy. I see about 25mpg - same as I get at 70mph in 6th. At 40mph I get 60mpg.

At 40-45mph (the typical 'city' speed limit here) more than half your energy goes into over coming aerodrag, so that's why I'd take lower drag. In theory you could drive slower, but in reality that would mean using a lower gear killing the benefit of slowing down.