Wazzajnr said:

No, what I'm saying is that the weight makes very little difference. UNLESS, as I've stated before it's hilly terrain. Or twisty roads.
Of course it takes more uphill.
I'll put money on it that the fuel economy of a 40ft curtainsider is less than 10% different between fully loaded and empty, but if you had a trailer full of bricks instead of a big trailer sticking above the cab, it would make even less difference.
UNLESS, I repeat, it's going up and down a lot of hills.
The amount of energy required to keep a mass moving along the road is tiny.

For example, on my ampera, at 80mph on a flat road with little wind, it needs about 20kw. If I open the windows it goes up to 25kw.
If I fully load it it makes so little difference it's almost impossible to see.
However, when I get to a hill, fully loaded it will go up to 60kw,empty it will be 40 on the same hill

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A couple of years ago going to Mancy airport to get a flight with three adults two kids and a boot full of luggage. I reset the trip computer on the move at the top of Brockholes brow going down to the motorway. How pleased was i to be doing 94.6 MPG at the bottom of the hill. Soon changed though, average there and back was 34 MPG. Unfortunately the world is not flat. But it would be helpful if it was all downhill.

wammers said:

A couple of years ago going to Mancy airport to get a flight with three adults two kids and a boot full of luggage. I reset the trip computer on the move at the top of Brockholes brow going down to the motorway. How pleased was i to be doing 94.6 MPG at the bottom of the hill. Soon changed though, average there and back was 34 MPG. Unfortunately the world is not flat. But it would be helpful if it was all downhill.

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No, but there are some areas that are much flatter than others. Brockholes brow is taking it to the extremes a bit though.

Tailwind back? The Boeing [P]387?!

Wammers should know all about tailwinds! Get in the Jetstream and you can throttle back and enjoy the ride.

Wazzajnr said:

No, but there are some areas that are much flatter than others. Brockholes brow is taking it to the extremes a bit though.

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Exactly and that is why some quote what others think are daft MPG figures. If you drive around Lincolnshire all day in your P38 you are going to get better MPG than someone who lives in this area where there are more bloody hills than the sound of music ever knew existed.

Grrrrrr said:

Tailwind back? The Boeing [P]387?!

Wammers should know all about tailwinds! Get in the Jetstream and you can throttle back and enjoy the ride.

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I get lots of tailwinds specially after a good curry.

wammers said:

Exactly and that is why some quote what others think are daft MPG figures. If you drive around Lincolnshire all day in your P38 you are going to get better MPG than someone who lives in this area where there are more bloody hills than the sound of music ever knew existed.

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Oddly enough there is a link to the Sound of Music over the hill from you a bit near Bolton, Robert Whitehead who invented the torpedo. Think his granddaughter or something married the Von Trappe bloke and gave rise to the family and singers the film was based on.

wammers said:

Exactly and that is why some quote what others think are daft MPG figures. If you drive around Lincolnshire all day in your P38 you are going to get better MPG than someone who lives in this area where there are more bloody hills than the sound of music ever knew existed.

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This is the thing, our P38 normally averages about 14 mpg because it's nearly all town driving. When I used it to collect the classic it was 95% motorway and I was quite lucky with the traffic as not too many holdups. I found that a lot of the time I was hardly pressing on the accelerator, and when I saw that a hill was coming up I would increase speed slightly to get some more momentum so that I didn't have to use as much power up the hill. I checked the computer mpg against the brim to brim fill up when I refuelled and the computer was pretty much spot on. I was surprised it achieved 22 mpg going and overall 18.7 but it just shows how you drive and having a well maintained car make a big difference. I have a Volvo S60 D5 185bhp, on a long motorway run I can do over 60mpg keeping at 70-80 mph which for an old school diesel 5 cylinder 2.4 I don't think is too shabby!