Of course hills etc are relevant. You referred to gravity not being relevant, but of course it is- that on of the forms of resistance (going up hill) and a positive when going down.
Absolutely agree that the greatest factor on fuel use is vehicle speed ( or as we are talking physics we should say velocity). The graph shown only applies to constant conditions, and very few roads have constant conditions, snd then of course it fails to consider the driver’s influence in a real world environment
Bearing in mind that at worst your graph is actually showing around 20% discrepancy between 90kmh v 100 kmh.
Engine load needed to climb a hill will be far more than the difference in wind resistance on the flat.If you can carry more energy from the flat onto the hill that’s a net saving in energy.
We are comparing same vehicles same routes.
By your logic the US would have kept the blanket 55mph limit let alone still being profitable in a 65 mph + speed regime.
I’m not disputing that. I’m disputing the claim that hitting the hill at a higher speed would result in better fuel economy. If you’re using power and not momentum following a downhill stretch then it won’t because your engine will be running at a higher RPM to generate the same power so will be using more fuel.
So how much more fuel (about) would the vehicle doing 64mph use?
Monetary savings have absolutely zilch to do with fuel usage.
If fuel cost one penny a litre, and drivers were paid £100 per hour we would have trucks running faster than F1 cars.
1 you haven’t even proven any fuel saving because there actually isn’t much if any fuel saving in the case of, at worse, an SFC curve that’s the same at 65 as at 55 and at best slightly higher engine speed but the same or even 20% more engine load and hitting a hill with speed made on the flat at lighter engine load v having to apply much more engine load and maybe also downshift earlier in the climb.
2 There is most definitely a loss if you have to turn down work because the driver has run out of driving time or will do.A truck hauling freight nowhere is using zero fuel but making zero profit either.
So you’re in top gear at maximum speed on the flat you carry that same speed and same gear onto the thill.How are you getting the same power at higher RPM ?.
Yes on older designs we might have downshifted to peak power rpm then let the engine speed fall back to peak torque.
No need for that now.Either way you’ve carried more speed, generated by less engine load on the flat, further up the hill before the need to apply more engine load.
Bearing in mind 20% more drag doesn’t equate to 20% more fuel consumption.
That’s why the US ditched the blanket 55 mph limit.Less work getting done by each truck means more trucks required
Normally its the slowest possible speed without excessively labouring the engine, in top gear. If a vehicle had a particularly tall top gear and/or poor aerodynamics, one of the lower gears might squeak it for better economy.
It would use Less at 64 mph , at least 1 mpg better than 56 mph.
Well if you say so then fair enough. You’re more experienced than I am. But I still think a like for like truck on a clear run would use more fuel at 64.
We did a test some wile back two vehicles left the yard at night from LU7 to Fleetwood Dock 212 mile up 212 mile back one at 56 MPH other one No Fuse so say 60+ the one with NO fuse was 1 mpg better than the one at 56 mph, The fuel tank was at the yard so it was full to full, NOT dash mpg.
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1 mpg better offset by the fines if / when caught with a pulled fuse 
It proved a point 52 MPH does Not save fuel? 60 MPH is the limit if you have the right gear you can run at 60 MPH lol
Ref fuse a blown one? surely you had some of them in the day? and a little commen sence to go with it.
Back in the day AAH run at 52 mph but had to pay overtime to do the Warrington Trunk? great saying there by the management most probably now working at Culina another Company trying to save a tanner by taking a shilling off there driver.
Meaningless test. Not the same vehicle, not the same load, not the same driver. The fact it’s not the same driver alone can account for a difference of 1MPG or more. I used to do a night trunk to London from Hull along with a guy in another wagon. Both setting off the same time, the same route, the same load. 2 wagons on the job and we’d interchange between the two. I used 20% less fuel than he did and I arrived at the gate at the yard in London just as he was pulling through it.
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Go hard or go home, fuel’s cheaper than wages.
People like you make me laugh. You think you’re so clever yet you’re the idiot chasing yourself up your own backside all day stressed to hell deliberately choosing to make yourself poorer. Every hour you finish earlier as a result of your charging around is an hour’s wage you don’t earn.
Not if you’re salaried.
One thing about Conor ,You have a very Sexy voice ? You talk like a [zb], Why do you think the Paddies keep them lit ? Its not because they are Thick, And would i be running motors at 60 mph if it was costing me more money [zb], You will not be passing any of mine at night and checking whether they are watching a Film ffs. I have forgot more than you will ever know about operating commercial vehicles , go and tell your mother how to suck eggs ( as for the comment to SDU idiots like you make me CRY ) you need to give your head a shake ffs, Thats why you work for a agancy because you are Unemployable you think you have done it all? yes you have F/A, As for being poorer No Sharp Suits here paying the wages? And i have NEVER ask anyone to do something i CANNOT / will NOT do, Keep your elbows in as we are coming by, you will not see the screen as they are all RHD plus the cab will be lit up? What colour do you like as it can be changed for you lol.
[Worst of the personal attacks removed as per site rules. L. ]
No stress here, on a kilometre rate the more kilometers that can be done in an hour, the more can be earned.
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Rubbish.
So say for example you have 2 night runs. Going from Manchester to Falkirk, one is hourly paid, one is salary. On a clear run and a capable truck who do you think will be back first?
If I was the hourly paid driver I wouldn’t even be trying to get there in one hit. However if I’m salaried at a decent rate I’d be cracking on when I can.
Completely different so no one’s an idiot.