I've seen mixed figures on various websites as to which is more economical. Some say 140 some say 170. I would have thought the 140 would be just slightly better as it's less powerful but I know that doesn't always follow as a 170's setup may be more efficient? I have a 140 and average between 52 - 60 mpg on my daily commute which is 130 miles round trip and is all motorway. I'd be interested to know what other people are getting?