That makes more sense.
However I know that different drivers on similar trucks can give very different mpg figures. If the drivers also swopped trucks that would make it a better test.
Very interesting.
Letting it run on downhills is obviously going to be better for fuel efficiency, but the speed the limiter kicks in wonât affect that.
I would argue that reaching the top of a hill going 56 shows a lack of understanding if fuel efficiency is the goal. The downhill will be more than enough to get the lorry back up to speed.
One disadvantage of running on the limiter will of course be that when it comes to slowing traffic, the extra momentum which is gained through use of fuel will be cancelled out by braking. If going slower, less fuel will have been used.
Another aspect is coasting in to traffic lights etc and planning ahead. How many times do you see a tanker going like the clappers and then having to slam on the brakes? It comes down to those driving the lorries wanting to drive them like cars, as has been said before on the forums many times.
As for what is more efficient on a straight run, 65 or 55, I couldnât really give a monkeys. I wonât be going over 60 because there is a speed trace on the tachograph and I want to keep my licence.
Running at 64mph instead of 55mph over a long trip like London to Glasgow will definitely increase your fuel costs. On average, you could see about a 10-15% increase in fuel consumption with the higher speed, especially with a loaded trailer. Itâs all about how drag increases with speed.
How do you reach the top of a hill at 56 if you were limited to 56 at the start of the climb.
@carryfast Well okay, letâs say a gradual incline with a summit, followed by a decline.
Have you not done the Eco driving CPC course yet? 
I have just done it and it depends on tyre pressure,vehicle design,and how fat the driver is
Iâm confused, where you show d/t, is that distance and time? If so, why is d and especially *t * not in your earlier formula ? Isnât that the US rather than the more Uk / euro version?
Do you have a source for it? Not sure where the drag/co-efficient has been obtained for the examples vehicle.
Not saying itâs wrong, just trying to understand its relevance and reliability.
I havenât. First thing I do in any unit is switch off Eco
you donât know what you are missin 
it went into depth of pricing this and that. It is not something I think about when a delivery that was not on the truck amongst 10 deliveries,even thought I had the delivery note. I thought I balls up. Turns out it was never put on the truck
In the first equation, it is for the air resistance. So, distance is not relevant.
The second set of figures I used are the conventional v=d/t velocity equal distance over time.
Only the speed being changed was being discussed in the OP. I showed the formula so it clearer that the resistance varies with the square of the speed, not linearly.
Because the vehicle is the same one, in the first example, itâs accuracy is actually irrelevant.
Although there are ways to calculate or look-up air resistance of simple conventional shapes from tables and charts, something as complex as a truck or other vehicles can only accurately be measured in wind tunnels etc.
A decline will generally be the same either way usually little if any engine load and speed controlled by engine braking and brake inputs.
While climbing an incline will generally end up at a slower speed than started it at.But carrying a higher cruising speed from the flat obtained by relatively lower engine load, onto a climb, will reduce the amount of engine load required to maintain an equivalent speed up the climb.
Thatâs how it seemed to work in the real world.
Bearing in mind lower engine outputs in pre limiter days and better flatter torque curves and therefore wider peak specific fuel consumption bands in more modern vehicles.
So maybe the advantages of âgetting a runâ at hills arenât relatively as noticeable now as then.
That sounds quite reasonable to me.
I can asure you at 64 mph less fuel will be used than running at 55 mph, Why ? Because we have tried it lol, Try it yourself for a month then YOU will know as i may not be telling you the Truth ,
but you need ro run two side by side to be sure as conditions can vary ie traffic / weather, at least 1 MPG better
London to Glasgow driver would have a lot more time to drink Tea / Kip lol
You need mid range engine power to do any good on fuel? Low power allways working hard, High power needs more fuel even running light, Mid range best of both worlds we found.
You are quite right about modern vehicles but it still apply to some extent hit the bottom of the hill at 60 mph you are half way up before engine starts to work, hit at 52 the engine is working hard at the first pimple lol .
It seems logical that losing speed on a climb costs less fuel than trying to maintain speed.
Yet if we don;t know the vehicleâs co-efficient, its area and guessing on the speed, really not sure what value the formula has and the answer from it. But then again it is the american version of the ISO - d instead of âsâ etc.
We arenât trying to put a value on the fuel use. We are just looking at what happens with an increase in speed.
Iâm in agreement with Alpine.Real world average consumption figures A to B trump everything.I know from experience that running at pre limiter speeds wherever possible didnât result in any fuel consumption penalty.
They also had customer service and driversâ hours advantages.
WowâŚitâs interesting to see just how many commenting didnât do physics.
Itâs simple physics. You have three external forces to overcome as you drive down the road, rolling resistance, gravity and wind resistance (drag). Rolling resistance and gravity arenât speed dependent so in this discussion are not a factor. Wind resistance however is massively speed dependent. As your speed increases so does the wind resistance. This means more power is needed to overcome that increase in wind resistance which means you need to burn more fuel. As wind resistance increases with the square of the speed and not linearly as your speed increases itâs not a case that at 50MPH you have X wind resistance and at 60 MPH you have X+20% wind resistance, you actually have much more than that.
56MPH was chosen as a speed to set limiters at and also as a standard for measuring fuel economy because itâs a sweet spot on the graph of wind resistance where the penalties for speed on fuel economy vs journey time are acceptable.
And for anyone claiming Iâm talking rubbish just think to what your fuel economy is like when itâs a very windy day and youâre driving into a headwind compared to what itâs like when thereâs no wind or the wind is behind you. You use a lot more donât you?
All this bleating on about hitting hills at higher speeds under power means more economy etc is also wrong given that your engine is more economical at lower RPM with a higher load than at higher RPMs as to produce the same power at higher RPM you have to use more fuel and if youâre doing a higher speed youâll be doing more RPMsâŚif anyone disputes that hereâs a nice video for you to watch from someone with a masters degree in engineering.
When to shift for best economy.
You can bang on all you like about âoh but my real world experienceâ but it isnât an accurate test. Youâre not testing like for like because there will have been multiple factors that will have changed between the two journeys even if you think theyâre like for like that can make a quite notable difference.
Thatâs why I couldnât understand showing the American formula when none of the data was relevant and certainly didnât lead to the numbers you quoted