graball wrote:
Back in the seventiies when the fuel crisis was on , they reckoned cars at the time were most fuel efficient at about 56MPH (i think that was the actual figure), which led to the blamket 50MPH speeed limit (on NSL) at the time ( I also believe, this is why motor manufacturers used to quote fuel consumption figures at that speed). Obviously cars have changed a lot since then (most cars only had 4 gears then) but I would still expect the optimum fuel efficient speed, on free flowing roads, to be around 60MPH.
I think most cars were petrol engined back then - and yes they've certainly changed a lot! Even as a student in the mid 1980s I can remember asking a thermodynamics lecturer if he ever though internal combustion engined cars would become efficient enough for it to start to become a problem finding enough "waste" heat to power the heater, and getting laughed at! Nowadays current diesels nearly all have separate combustion heaters because they can't keep their own engines warm at low power - let alone the occupants! I've got a trip computer with instantaneous and average fuel consumption. It's usually on "average". I do a regular trip from rural Cumbria to Glasgow. That's 35 miles of NSL SC, 5 miles through the centre of Carlisle, 90 miles on the M74, 6 miles on the M8 and a couple of miles on 30MPH urban roads. I find the average consumption is about 38 by the time I hit Carlisle, and much the same by the time I hit the M74 (I go through Carlisle very early in the morning so traffic isn't stop-start). It drops to about 36 on the M74 (always fast and free-flowing) but then much to my surprise, it goes up again, slightly, on the M8 - despite it being pretty awful traffic!
As an experiment, I then put it on "average", zeroed it and drove at a constant speed for half a mile or so. (not on any of the above roads!) At the end I'd turn round and go back the opposite way to average out gradient and wind speed. I tried that at various speeds and found that about 20MPH gave me somewhere in the high 50s to the gallon!