Kevin pointed out that my tweet which said:
According to national geographic this mth, "average american car gets 21 mpg". Omg, my 2L clio sport gets 31, and I drive like an idiot.".is misleading.
According to this authority (picked at random from the net of nonsense and with no check by me) what we're saying is the US cars average 25.219946996999997 UK MPG. 
And my clio only gets 25.812901195923956 US MPG. Not so smug now, am I?
 How precise! But don't forget kids, as Uncle Danny never tires of saying, precision is not the same as accuracy.