MPG Vs MPH

newmercman:
Testing has proven that, with all other things being equal, a 1mph reduction in speed will bring a 1/10th mpg fuel saving on aerodynamic efficiency alone, add the other factors and dropping from 56mph to 50mph could get you an extra mpg. I know some of you have experienced far greater savings, but there are so many variables, wind direction, the friction coefficient of the road surface and a million other factors, that you can’t work out savings off the dash readouts, sure it’ll give a rough idea, but you’ll need to drive everywhere at the reduced speed for at least a week to get an accurate picture :wink:

Spot on Mr Lee :smiley: As you say, you really need to discard all the variables before making any definite MPG claims, although knocking back the cruise a couple of clicks is never a bad idea. Our firm’s policy is 88km/h, even though he truck limiters are left at 90, and over a fleet of around 1000 artics it has proven to make a difference.

One interesting tip that I picked up at a Merc’s driver training facility in Wurth is that there’s little point trying to gently accelerate up to cruising speed. They found that to acheive any set speed on a level or inclined road requires X amount of fuel to do so, whether you tickle the truck along gently up to pace or floor it, provided you keep the engine revs within the economical sector there was no perceptible difference. With that in mind I have no qualms about just pressing resume on the CC when accelrating away from peage stations (letting the auto-shift do the work of course - the days of Fuller boxes or the nasty Renault B18 are pretty much a distant memory over here!)

~ Craig

My old engineering tutors always used to impress on us that you can have - MPG or MPH - you can’t have both. :slight_smile:

Craig 111:
Spot on Mr Lee :smiley: As you say, you really need to discard all the variables before making any definite MPG claims, although knocking back the cruise a couple of clicks is never a bad idea. Our firm’s policy is 88km/h, even though he truck limiters are left at 90, and over a fleet of around 1000 artics it has proven to make a difference.

One interesting tip that I picked up at a Merc’s driver training facility in Wurth is that there’s little point trying to gently accelerate up to cruising speed. They found that to acheive any set speed on a level or inclined road requires X amount of fuel to do so, whether you tickle the truck along gently up to pace or floor it, provided you keep the engine revs within the economical sector there was no perceptible difference. With that in mind I have no qualms about just pressing resume on the CC when accelrating away from peage stations (letting the auto-shift do the work of course - the days of Fuller boxes or the nasty Renault B18 are pretty much a distant memory over here!)

~ Craig

Spot on Mr Sheer :smiley: I picked up that very same tip from Volvo’s training facility, you can’t save fuel trying to get 40 tons to crusing speed so don’t try, just keep her in the torque and get it over with as soon as possible, the electronics are supplying the optimum amount of fuel to the engine so keeping the right foot horizontal doesn’t drink fuel as it would in a lorry with a mechanical fuel pump :wink:

Autoshift and Cruise, what a fantastic combination, throw a retarder into the mix and you’ve got a nice easy drive wherever you go, nothing like coming off a roundabout at the bottom of a big hill, hitting resume and letting technology do all the hard work for you :sunglasses:

took this up the other day, was getting 4.5mpg, so pls tell me how i can improve the mpg, i only run at 50mph tops too, perhaps i need to move the trl closer…lol

full veiw, whilst unloading

If you load the silo the other way around, with the skirt facing forwards, its like driving with a wind sock behind you, and the fuel consumption gets even worse, i have had 3 mpg before…

I took this from Derby to Cullompton in Devon on Friday. I kept her at 50 all the way there and all the way back (came back empty) and it averaged 9.7mpg. I necked it before I set off and did the same when I got back and she held 185 litres. I wish it would do that on tipper work. Sam

I get more per gallon than most drivers over here because I drive european style and therefore more economicaly. I can get at least 1 mile more per gallon.
Drivers here drive so hard, foot down to the floor in every gear and red lined each gear change.
Slowing down does not always save fuel, we had our trucks cut from 69mph to 65 mph and fuel consumption went up because we couldn’t get the same run at the hills and instead of getting over without changing down we now have to drop a gear or even two.

Surely if you put 2 trucks on the same route same deliverys, one running at 56mph and one at 50mph. And let’s say it takes A full 9 hrs drive time for the one running at 56 mph and 10 hrs for the one at 50mph to complete the work. Then surely any fuel savings are gone as the slower vehicle is running for an extra hour therefore using more or the same fuel as the one at 56 mph. Just a thought. :slight_smile:

podge:
Surely if you put 2 trucks on the same route same deliverys, one running at 56mph and one at 50mph. And let’s say it takes A full 9 hrs drive time for the one running at 56 mph and 10 hrs for the one at 50mph to complete the work. Then surely any fuel savings are gone as the slower vehicle is running for an extra hour therefore using more or the same fuel as the one at 56 mph. Just a thought. :slight_smile:

plus wages, and the one at 50 mph loses 6 hours per week turnover.

podge:
Surely if you put 2 trucks on the same route same deliverys, one running at 56mph and one at 50mph. And let’s say it takes A full 9 hrs drive time for the one running at 56 mph and 10 hrs for the one at 50mph to complete the work. Then surely any fuel savings are gone as the slower vehicle is running for an extra hour therefore using more or the same fuel as the one at 56 mph. Just a thought. :slight_smile:

Its mile to the gallon, time is not a factor, distance and gallons are the only factors.

Sounds plausible, that if you go quicker, then the engine’s running for less time, therefore it’s more economical, except, as explained above, to travel twice as fast you need 4 times as much fuel to overcome wind resistance. Comparing 56 to 50, 56x56=3136, 50x50=2500, 3136/2500= approx. 1.25, therefore the faster speed is c. 25% more expensive on fuel (all other things being equal). 1 more hour is c. 11% more time 9 hours to 10 hours, so to “waste” 11% time you “save” at most 25% of the ever more expensive juice. Assuming you cover 500 miles in a day (!), then the faster speed might use 50 gallons say (optimistic I know), the slower would then use 40 gallons. 10 gallons = £60? So unless you are paid more than that per hour (super super tanker driver?) the lower speed (up to a point) is more economical, not taking into account all the other factors. As for the big tube and the big square being transported in the above photos, there’s nothing to be done in terms of aerodynamics; but driving slower yet still keeping it in top gear should be more economical on fuel. I would be interested to hear the detailed overall savings made by Tescos running at 80kph or whatever it is. I challenge any of our regular readers to prove my figures wrong. Shouldn’t be too difficult.

I’ve been driving two identical TGX440’s on container work. One unit returns anything up to 11.5mpg the other struggles to reach 10mpg. Same work, same driver.

Get rid of the m.a.n 440s they are a terrible truck.I find they wont change into top gear on cruise at 40 running at 44t.Constantly changing up and down gears now they have dropped it to a 9 litre to get around the adblue.The trucks having to work harder.Get your drivers to use cruise as much as possible and not to accelerate or brake hard.Air kits work but only if pulling the rite trailer. Waste of time on my merc as i pull a bulker.If i fanny about i can get 13 mpg empty running at 45mph and the best i have had it loaded is 8.4mpg.I average anywhere between 7 and 8.2mpg as a rule.

Two things with regard to above postings, first the actual time taken to travel between two points for a truck doing a maximum of 56mph against one doing a maximum of 50mph is so small it makes no difference, on paper you may lose 6 miles every hour, but in the real world you’ll only be 10mins or so behind at the end of a shift.

Secondly, it doesn’t matter what speed you are doing, if you’re in top gear the engine will turn the same revolutions to do a specific distance, if the engine turns faster the distance will be covered quicker, but the total number of revolutions will be the same. Although if you race through the gears to get to the higher speed the engine will turn more revolutions at the higher speeds going through the gears, so total RPMs for any given distance would be higher, but for a measured mile starting and finishing at your maximum speed the RPM total will be the same.

AHT:
If you just want to get good MPG then basicly the slower the better given that most of the energy used is wasted as air resistance (providing you are with in the efficient working limits of the mechanical parts) so probably around 30-40mph,
In terms of family car drag coefficients the fuel consumption increses exponentialy with speed, once you reach speeds above 50mph the fuel consumption increses drasticaly.
For a truck with the far less aero dynamic shape i would say the tipping point is about 40mph
the formula for drag is below
Drag force = (1/2)p(v^2)Cd*A
As you can see speed v is squared hence the exponential increse
Cd is the drag coefficient witch is dependant upon the shape

(I am a mechanical engineering student BTW looking for HGV work to fill up my spare time (and because I enjoy driving))

OW ! My head hurts now… !
(I am a frustrated professional footballer, but driving HGVs cause i didn’t listen at school)

SwedishSteel:
I am razzing around in a chipped and remapped volvo FH that is limited to 59 mph and always pulling 44t. Just over 6mpg average, my boss doesnt seem to mind as long as I get all my loads done without cooking the engine!

Either your not very good at driving economically or the truck has had it well unless you pull a low loader!