I've never known a car that can change mpg figures quite so drastically, admittedly I've always used cheap supermarket fuel, not for the sake of being tight, but simply the fact I drive past Morrisons petrol station 6 days a week, I'd have to go out of my way to go to a proper brand petrol station, my job normally entails at least 15 miles to get there ,sometimes more, sometimes less, but normally more,a couple of years ago I had a job about 9 mile away, lots of very bendy roads and not many straights, the car hovered around the 40 mpg mark all the time I was there (about 8 months) <_< ,I had a job last year that was 35 miles away, 4 miles of small road,31 miles of dual carriage averaging around 70 going and about 65 on the return, car then averaged 52-54 mpg for the month I was going down there.
Had a weeks holiday of normal driving but keeping the speed around the 60 mark, one long journey, 120 miles each way, then a few more local days out, managed 56.5 mpg from that tank full, actually got to 624 miles on the trip meter before it got to no miles left on the trip computer, but it still had fuel left in it.
The best I ever got from it but could not measure,I only had the trip computer as a ref was a 73 mile journey each way of virtually clear dual carriage way, I set cruise to 65 mph on the way up there with a reported average of 62.5 mpg, then without resetting the trip I came home at the dreaded official speed quoted on most car fuel figures , "56 mph" locked into the cruise and stuck to for 70 miles, the average then went right up to 68.9 mpg and was still slowly rising all the time but I had to stop the journey, so with no proven figures on that run , it must have been somewhere in the mid 60's.
On general driving the average fuel guage is usually wrong in my favour, on one tankfull it reported an average of of about 51 mpg but actuall figures were 53 mpg. All my figures are measured amounts no guesses, I fill up to the brim each time, then go to work immediately so the fuel doesn't drain out the breather pipe. That way I know its the same total fill up and correct amount of fuel used every tank, I don't do it much now as I know roughly what the car does now. The car hardly ever gets town use, that would probably give poorer than the 40 mpg that are my worst figures.
so just to sum things up
Car Ford C-Max 05 1.6 TDCi Ghia 110 bhp (now at about 55k)
Fuel Cheap Supermarket Diesel
Airfilter Standard Ford
Worst 40 mpg
Average 45-46 mpg
Long run 53-56 mpg
Best Careful 65 mpg
Sorry such a long post but accurate figures from my car with accurate fuel amounts used, we all know there's a large variation of amount of fuel you can get in your car in the different conditions between hot and cold, somedays the fuel goes in fine, then certain days it froths up like anything and trips the fuel pump, the tank isn't full but the froth stops more from going in, at least with my brim top fill ups I know I had the correct amount of fuel measured each time :D :D
So there's your staggering difference from worst 40 mpg right up to Careful Best of about 65 mpg
That's only a difference of about 25 miles per gallon, :( :( ,not 25 miles per tank full :angry: