I was in the Volvo dealer last week and while there asked them to recalibrate the speed limiter on an fh. According to a Garmin sat nav it just does 51 but the tacho says it is doing 86 km.The fitter said the tacho was the more accurate, where as i would think the sat nav which is after all GPS would be more accurate. Who’ s right.
At a continuous speed the satnav will be more accurate.
tallyman:
At a continuous speed the satnav will be more accurate.
True.
But remember its your tacho the VOSA bod would use for evidence 
My premium said 56 on the dash, but the navman tracker/satnav flicked between 52 and 53. It wasn’t able to do some of the long trunk runs that other trucks could do because drivers ran out of time, proving that the GPS speed reading was probably more accurate. Didn’t bother me cos I only do local stuff. It’s now been turned up and does its 56, but I’ve noticed that it now uses a lot more fuel compared to when it was slower, which was probably bound to happen. So unless it’s not costing you too much time, I would leave it as it is.
We have 2 identical fh 500’s. One does 51 on sat nav and the other 53 but both read 86 on the tacho. The problem is the night trunk men fight over the one that does 53. Volvo said they couldn’t do anything because head office has given instructions that that is what they are to be set at and as far as they are concerned they are both set exactly the same.
By law the tacho cannot be more than 2% out. There is no law covering your Satnav and civilian Satnav uses a far less precise system than is available to military and government.
Calibrate a tacho/speed limiter on an old set of tyres on the drive axle, then stick a new set on and youv’e got a flyer.
schrodingers cat:
Calibrate a tacho/speed limiter on an old set of tyres on the drive axle, then stick a new set on and youv’e got a flyer.
Tachograph says 89
Sat nav says 57mph, I say bye bye to most other trucks 
Conor:
By law the tacho cannot be more than 2% out. There is no law covering your Satnav and civilian Satnav uses a far less precise system than is available to military and government.
I’m with Conor on this one. In laymans terms civilian gps has a built in “wobble” to it. It’supposed to stop Mr Al-kaboom putting a home made missile through the White House conservatory.
Ex Haulier:
I was in the Volvo dealer last week and while there asked them to recalibrate the speed limiter on an fh. According to a Garmin sat nav it just does 51 but the tacho says it is doing 86 km.The fitter said the tacho was the more accurate, where as i would think the sat nav which is after all GPS would be more accurate. Who’ s right.
The limiter and speedo has not been calibated accurately - simples
GPS is supposed to be thee most accuarate way to determine speed, the “tacho” (speedo) is only as accurate as the calibration
Sloppy calibration seems to be more and more common these days
My truck was way out from brand new and as the tyres started to wear it became worse
Mine had to be re-calibrated as it was so far out and now it does 90 kmh and I seem to go past 80% + odd of other trucks, now that it is accurate, Sat Nav also says 90 (when set in kmh)
damoq:
My premium said 56 on the dash, but the navman tracker/satnav flicked between 52 and 53. It wasn’t able to do some of the long trunk runs that other trucks could do because drivers ran out of time, proving that the GPS speed reading was probably more accurate. Didn’t bother me cos I only do local stuff. It’s now been turned up and does its 56, but I’ve noticed that it now uses a lot more fuel compared to when it was slower, which was probably bound to happen. So unless it’s not costing you too much time, I would leave it as it is.
The reason for that is most likely it was reading more miles than it was actually doing - now its accurate it says less, making your mpg look worse
thelorryist:
schrodingers cat:
Calibrate a tacho/speed limiter on an old set of tyres on the drive axle, then stick a new set on and youv’e got a flyer.Tachograph says 89
Sat nav says 57mph, I say bye bye to most other trucks
Mine same…although I use cruise at 86 until needed
Bear in mind the Garmin works by triangulating your position using at least 3 satellites in geostationary orbit over the equator. They judge your speed relative to your position in relation to them. They actually don’t account the vertical distance you travel (up and down hill), so tend to underestimate your true speed over the ground.
Your tacho on the other hand measures the speed your wheels are rotating and translates that into the time against distance you are travelling over the road.
If it’s accurately calibrated then it will be more accurate than the sat nave ever can be…but it’s impossible to calibrate 100 per cent accurately 100 per cent of the time because there are variable factors (the circumference of the tyre is reduced as it wears for example).
I was going to say that I hope that this helps…but I’ve just realised it’s no help at all.
Get a helpful policeman to pace you using a traffic car: their speedos used to have to be calibrated very regularly (daily or weekly, I think) or they couldn’t make speeding convictions stick. I don’t know if this is still the case, but old traffic cars used to have an additional huge calibrated speedo tacked on the dash where both driver and passenger could view it.
Something to read whilst waiting around
Ex Haulier:
We have 2 identical fh 500’s. One does 51 on sat nav and the other 53 but both read 86 on the tacho. The problem is the night trunk men fight over the one that does 53. Volvo said they couldn’t do anything because head office has given instructions that that is what they are to be set at and as far as they are concerned they are both set exactly the same.
You’ll need to go to a Tacho Center to get the Tachograph recalibrated, as this is where the limiter takes it speed signal from. Make sure you’ve a set of well worn tyres on the drive axle though