Can someone please explain this

I do the same run every night, I go exactly the same way, stop in exactly the same places, turn in the yard, jack it round, back onto a bay, drop trailer, swap trailer, etc.

How come when I drive a '05 merc actros the end mileage is 4 kms less than an '08 MAN■■?

My end mileage in a MAN is about 556kms. In the merc it is about 552.

I know the speed is not calibrated, I set satnav (yes I know I don’t need it but just to watch the mileage go down!) for 56mph, time arrived is always 5-6 mins after the satnav tells me at the start of the run. I know lorries do slightly less than the speedo says, but the mileage■■?

Anyone know why? I’m just curious about it. I asked a driver tonight and I should have known better than to ask this particular one, “Ah, that is because the MAN is faster, you are pulling out and overtaking a lot more, when you pull out then back in, it adds 4 kms to your run.” I didn’t say anything, just looked at him with the ‘don’t take the ■■■■’ look! The merc is actually a little bit quicker than the MAN!

Is it doing those ‘Wheel spins’ from a standing start that is making the difference :question: :unamused: :wink: :laughing: :laughing:

Seriously, could it have something to do with wheel or tyre size :question: :question:

A number of reasons:

  • Tyre pressure - the higher the pressure, the larger the overall effective diameter of the wheel, and therefore the less rotations it will take to cover the same distance, giving a lower reading on the tacho
  • Tyre wear - the more worn the tyre, the lower the effective diameter, and the more rotations to cover the same distance => higher km reading
    Consider a tyre with a diameter of 1 metre when new. The circumference (the distance it will roll with one revolution) is Pi x diameter.
    If 1 cm of tread depth is then worn off the tyre, the diameter is now 98 cm (you’ve lost 1 cm off each end of the diameter, so the diameter has been reduced by 2 cm.). Therefore, the diameter has been reduced by 2%, so the circumference has also been reduced by 2% (since the circumference is proportional to the diameter), and the reading you get on the tacho will be 2% higher.
  • Temperature - affects tyre pressure
  • Lanes used - for example, if you’re in the middle lane on a right-hand bend, then you’re going to travel a slightly shorter distance than if you’re in the left-hand lane.
  • Calibration - tachos don’t have to be calibrated to the level of accuracy that you’re implying. In fact, it’s impossible to keep them calibrated to that level of accuracy, due to the tyre wear variations stated above.

The difference you are seeing is less than 1%, and is well within the tolerance required by tachograph calibration. It could easily be accounted for by the factors listed above.

Just like mr flibble said.

Always get your truck caliberated when the tyres are nearly bald.

are both trucks exactly the same length

I think youve got too much time on yourr hands
and need a multi dropper day job :laughing:

the merc corners better than the man,so you must be throughing it ito corners and rounabouts,which means you use the raceing line like hamilton in his race car to go shorter distance.

Just like mr flibble said.

Always get your truck calibrated when the tyres are nearly bald.

shush Man. You’ll have everyone at it :wink: Don’t do it if you’re on mileage or economy bonus mind.

In addition…the satnav is defaulted at car speed limits. You’ll get a more accurate arrival time by changing the ‘limited speed’ function…(if it has one)
I set the ‘limited speed’ page to 48mph, that way i’m always ‘early’ by a few minutes!!!