Truck pulled in front of me last week showing 64mph on the radar thing in mercs., on the flat just before Hilton Park. What would be the difference in fuel cost over say a London to Glasgow run? Running at that speed compared to 55?
Had me trying to work out some sort of figure all the way home 
Edit. Assuming pulling a fully loaded trailer.
Very roughânâready: the speed has increased by a bit under 10%, so the air resistance will increase by a bit under 20%
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= | drag |
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= | density of fluid |
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= | speed of the object relative to the fluid |
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= | drag coefficient |
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= | cross sectional area |
So, of the order of 20% more cost.
As a quick explanation
The mpg is dependent on fixed and speed variable resistances. The air resistance is dependent on the square of the speed. So a 10% increase means the new speed is 110% of the original speed. Squaring up 110% gives about 120%.
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So you obviously know your stuff
Just curious then what would be the best speed to achieve the most mpg then?
Bearing in mind air resistance is also dependent on wind direction and strength and trailer/load height.
Night of the '87 storm still had a strong tail wind blowing at 60 mph + Scania 112 Northbound on M25 from Heathrow very little throttle needed in top.
Southbound on M1 needed to drop gears on the flat and downhill flat out and was lucky to maintain 50 mph.Mirror arms struggling.
In North America truck speeds on Interstates are/were generally more than 70 mph.Even at higher Canadian weights.
Real world figures seem less pessimistic than theoretical ones.
Before limiters was doing very similar speeds to that over large sections of around 400 mile round trips night trunking.DAF 2800 among others around 7.5 mpg average.
Bearing in mind modern engine technology improvements.
How can any truck maintain that speed here now with a limiter set at 90kmh unless itâs North America.
If you want to get that technical then you can add moisture density, heat, elevation, amount of insects murdered per mile.
Seriously though these things would make little difference on an averaged out route, ok if you were driving into a continuous head wind for 100 miles as opposed to being pushed by a tail wind then yes youâd notice a clear difference, anything else is just drops of diesel difference.
I do always feel (and by that I do mean a whimsical feeling) that my freshly waxed and polished car is more fuel efficient than when sheâs a dirty gertie.
For any given vehicleâŚthe resistance is affected by the square of the speed.
Smaller, slippier, vehicles will have less air resistance than bigger, bluffer ones, obviously.
Air speed, (wind) is a factor.
For simplicity ignore the side wind component and look only at the head/tail wind.
And again for simplicity assume that the vehicle is going out and returning with the same wind conditions. ie any head wind in one way is tail wind in the other.
If there was a linear relationship between air speed and resistance then the two runs would cancel each other out. But it not a linear relationship, as pointed out it varies according to the square of the speed.
So for simplicity letâs choose nice round figures.
10 d/t truck speed. A run of 10d each way.
And one calm day, and another with a 10 d/twind blowing straight down the road.
Day one, 10d/t âheadwindâ for 20 d. all else being equal the resistance due to speed is 100 units for all the trip of , the fuel is 2,000 units. (10x10x20)
Day two, the has no headwind for one run. It is travelling at the same speed as the wind. So no fuel is burned in overcoming air resistance. The engine merely ticks over, plus a bit to roll the tyres.
The opposite direction the truck has a headwind of 20d/t. So the fuel is 20 squared by 10 units only. (20x20x10) 4,000 units.
So a very windy day will make a huge increase in fuel use, even on a round trip.
Engineers have curves that describe the engines specific fuel use.
That is the engines use fuel more efficiently at certain rev ranges, and that is what as marked as green on most tachometers. The trucks are generally geared to be âin the greenâ at the normal running speed, and thatâll be at 90km/hr for most UK and EU spec trucks.
As an aside I had a back axle/diff break when in Spain. The dealer said that the truck was UK spec and the dealers in Spain had none the correct ration so the factory had to send one out. At that time Spanish trucks were on 100km/hr limiters, not 90. I would have been happy with a higher diff, but the dealer/factory wasnât having any of it!
Weâre just comparing like for like vehicle types at given speeds.
The difference between post limiter 90kmh max v pre limiter 60-65 mph max was negligible if anything.If anything the slower approach speeds to hills meant higher engine loads and higher engine speeds and lower gears needed before reaching the crest.
Best specific fuel consumption correlates with around peak torque.
Thatâs where a big power truck can win out over a lower powered truck.
Because itâs peak torque figure can be closer to the peak power figure of a lower powered truck.
Possibly to the point of being as, if not more, economical at a higher speed, than a lower powered truck especially on hills.
Especially true in the case of small capacity engines which make power by high engine speeds rather than more torque at lower engine speeds.
In all cases running at higher speeds on the flat and carrying that speed onto hills saves fuel.
So about ÂŁ40 more fuel cost give or take.
Hi. This is a FACT running from Leighton Buzzard to Fleetwood Dock which is 212 miles so 424 miles round trip pulling P&O crap trailers one vehicle 56mph the other No Limiter was ONE MPG better than running at 56 MPH 4.25 hrs running time, So the answer is 60 mph better fuel than running at 56 mph its still like that today 
So when Paddy comes by you he is the one thats switch on, not the silly C===S running at 52 MPH abusing the vehicles,
Remember 60 mph is the limit some people out there dont like to much over that to save your fuel lol,: 64 mph about right though 
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You are spot on with that theory, we have done that test many moons ago 60/65 saves fuel over 56 mph for that reason , as for the â â â â â running at 52 mph must have Chimps operating the show,
Let him waffle, it makes him feel knowledgeable and superior. Poor deluded fellow.
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So say 5 of those runs in a week thats ÂŁ200 more in fuel. How can that pay?..
Ironically Iâm small part Irish.A decent engine rumbling nicely along at around 65 mph felt right especially the old DAF motor and never saw any real world advantage at 56 max.But hitting the start of hills at 56 max, that previously hit at 65, was an absolute joke.
Luckily most of my time was pre limiters.
A company I donât wish to mention turned their fleet down to 52 about 4 ~ 5 yrs ago, they put them all back up to 56 around 6 months later and I heard (unverified) the person that made the recommendation had to seek other employment.
I donât doubt the FACT (after all it is in caps) that on a run with two different vehicles, and two different trailers there would be a difference of 1mpg or more, and that a faster vehicle might have been better.
Repeat the experiment with the same vehicles and drivers, with the same trailers (not âcrapâ possibly with soft tyres and dragging brakes) and do it on multiple occasions and I would be very surprised if the same set up at 65mph was better on mpg than one at 55mph.
When we were running unlimited Gardner and Cummins, later Volvo and Merc, the company put speed limiters on some of the trucks. This is pre-legislation and they were set at 60mph. They experimented on half a dozen trucks, all did better mpg so they did the whole fleet.
Ref dragging brakes ? P&O Pandora trailers with brakes that would be a first lol Why did VOSA shut them down in the end ?
By the way this was not done on one run, it was tested over many night runs to the dock, at the time with two new IVECOs.
Today we operate DAFs on dock work the one`s set at 60 mph are better on fuel than 56 mph all though all pulling mega trailers max payload being 10/15 tons.
back in the P&O days max weight on tandams not triaxle so less drag but set runs ie Midways Depot to Fleetwood dock 212 miles both vehicles ran to gether so same conditions, fuel was on tap in the depot so no detour? (There where options to make mph very good though lol) but not used when doing our fuel tests
Thats half the reason for better fuel you where more than half way up the hill before the engine needed to start to work lol.
When you are the man paying for the fuel you soon work out the Best way to save money ?
I all ways wondered what the Paddy keep he Lit ment? now i know and people think Paddies are thick? Not sure about that, more like the 52 mph idiots me thinks, in fact i know, No point in getting old if you dont get wise just remember that lol