42 miles per gallon = 17.8560357 kilometers per liter. According to Google. Sounds very doable. Your average Jap city car these days, already gets that, and more.
My 20 year old VW Golf Diesel goes 22 km/liter (and the motor is pretty worn out), so i really think that new cars should be able to top that. The Mini Cooper Diesel goes 25.6 km/liter in EU mix (thats both urban driving and freeway driving) and it is not slow either.
I've got a bad feeling about this.