Terry T:
Not quite. I did a real world test on this earlier in the week. I had to collect the same load 2 nights on the trot from London and deliver to Birmingham. Not pushed for time and hours on the turnaround so had time to waste. Not sure on the exact weight but it was definitely the same weight for both trips. Totally empty we get mid 13s and full up (not necessarily max weight) we get low 9s on the limiter so make of that what you will.
There was a lot of road driving before I hit the M40 so I decided to reset the counter once on the motorway as the destination was only just off the motorway the other end. But it was dark and raining so I accidentally reset it wrong (2 mile out) on the second trip but it’s close enough to get the gist.
90km/h - PPC set at +4 & -3. Had to override it a few times to prevent an overspeed.
80km/h - PPC set at +10 & -3. No need to override as engine brake prevented all overspeeds. Second trip average only 2km/h lower than set cruising speed but first trip 4km/h lower. That’s a surprising amount of eco rolling on the second trip.
At 56 I hit 10.7mpg and at 50 I hit 11.6. That’s a 0.9mpg saving. Sounds impressive at first glance but there are other factors at play here. Firstly, the 50mph journey took approximately 12 minutes longer. Add another 12 minutes for the return journey and that’s another 30 mins over time for me as I’m paid by the hour. So while the company might have saved nearly 4 litres of diesel each way on the slower run, any money they saved was paid to me in wages. And then some.
It really depends on circumstance. If your drivers are salary paid and the load isn’t on some strict time schedule then I guess a slower speed can make a difference. And there’s always the carbon footprint bragging rights. But for hourly paid drivers racking up 100s of miles a day I’d say the faster the better. Within reason obviously. Would like to retry it at 60mph. Maybe after Brexit I will get the chance.
One thing I did notice was the increased amount of great changes at 50mph. At 56mph the box will hold on far longer going up hill before changing down.
Actually this fits in beautifully, what i believe @franglais was trying to say is a 10% increase in speed will produce a 21% increase in wind resistance. However (and you can see the post I made earlier up thread) wind resistance is not the only game in town, at around mid 50s mph, wind resistance accounts for about 38% of fuel, rolling resistance 38% and other wastages about 24%
So lets look at your figures, when you increase your speed by 10% you used 8.4% more fuel
an increase in speed of 10% would give an increase of 21% wind resistance. Your rolling resistance and other wastages would remain the same so only the fuel needed to overcome wind resistance would increase. So we are only adding 21% to 38% of the fuel you used which would work out at an expected increase in 8% more fuel needed
On a more detailed breakdown…
On your slower journey you used 9.54 galllon, this could be broken down to
3.63 gallon for wind resistance(38%)
3.63 gallon for rolling resistance(38%)
2.28 gallon for other wastage(24%)
a 10% increase in speed would add 21% to only the 3.63gal used to overcome wind resistance = 4.39 gallon
add the other two figure back on
4.39 + 3.63 +2.28 = 10.3 gallons
you used 10.34 gallons
now isnt that astonishingly close to the figure you proved in a real world test.
you
) Cubic yards mean nothing to me whatsoever and I hate it when builders ask for shingle in imperial like 3/4 gravel.
