Does anybody on here with a TomTom sat nav find that the speed on the sat nav is lower than the speed on your tacho?
I operate a Scania which reads 91 km on the tacho but the TomTom says the truck is only doing 54mph.
The truck has been into Scania to be turned up which resulted in a new speed limiter sticker being applied with the set speed now up to 90 kmh.
This means that before it went in it was only doing 53 mph/85kmh as per the old limiter sticker but the tacho still read 90 kmh.
If you sit at speed limits 30/40/50 TomTom still reads 3 mph slower than the tacho.
Does anybody have the same senario or have I done something to me sat nav ?
My TOM TOM is exactly the same. It is reading about 3 mph slower than the speedo reading. I would imagine it is because most speedo’s read the speed higher than actual road speed. The TOM TOM is calculating your speed off GPS so it will be pretty much accurate. Although mine was playing silly buggers the other day and told me I was doing 18000 mph for a few seconds! (Thats right 18000 mph)
Scania’s are notoriuos for going slower than they should on the limiter, and usually get worse with age too. 
the gps speed is correct as the tacho has a 0 to 10% discrepency the gps is accurate to 0.1mph when at ta steady speed on flat ground so the gps speed will be correct
I actually noticed the same thing today, Only difference was I was in my car doing 70 and my TomTom PDA was telling me I was doing 66mph. But then tyre wear etc. has alot to do with speedo readings being out.
My Tom Tom Go also reads about 2 to 3 mph slower than the Speedo reading.
MrReliable:
Does anybody on here with a TomTom sat nav find that the speed on the sat nav is lower than the speed on your tacho?
I operate a Scania which reads 91 km on the tacho but the TomTom says the truck is only doing 54mph.
The truck has been into Scania to be turned up which resulted in a new speed limiter sticker being applied with the set speed now up to 90 kmh.
This means that before it went in it was only doing 53 mph/85kmh as per the old limiter sticker but the tacho still read 90 kmh.
If you sit at speed limits 30/40/50 TomTom still reads 3 mph slower than the tacho.
Does anybody have the same senario or have I done something to me sat nav ?
My english isn’t really the Best 
But,your Tyre are Rubber,blowen up whit Air or Liquit.
On Truck’s by Air.
The Tacho is celebrated by a empty Truck whit new Tyre.So older the Tyre are,and so more they are pressed down by your Load,so more often they have to turn around to get 56 Miles in one Hour.
So,whit a full Load will you be underway whit 54m/h,as the Computer say’s.
Radiusx2 =x3,14=
40cmx2=80x3.14=251,20
90 km =90000 Meter
1 Hour is 3600secound
90000:251,2=358
the wheal goes 358 times around to get 89,9 km/h
if your Tyres old,and you are loaded will you have 37x2=74cmx3,14=242,36
242,36cmx358rpm=86564,88km/h
fred Kanka:
MrReliable:
Does anybody on here with a TomTom sat nav find that the speed on the sat nav is lower than the speed on your tacho?
I operate a Scania which reads 91 km on the tacho but the TomTom says the truck is only doing 54mph.
The truck has been into Scania to be turned up which resulted in a new speed limiter sticker being applied with the set speed now up to 90 kmh.
This means that before it went in it was only doing 53 mph/85kmh as per the old limiter sticker but the tacho still read 90 kmh.
If you sit at speed limits 30/40/50 TomTom still reads 3 mph slower than the tacho.
Does anybody have the same senario or have I done something to me sat nav ?
My english isn’t really the Best
But,your Tyre are Rubber,blowen up whit Air or Liquit.
On Truck’s by Air.
The Tacho is celebrated by a empty Truck whit new Tyre.So older the Tyre are,and so more they are pressed down by your Load,so more often they have to turn around to get 56 Miles in one Hour.
So,whit a full Load will you be underway whit 54m/h,as the Computer say’s.
Radiusx2 =x3,14=
40cmx2=80x3.14=251,20
90 km =90000 Meter
1 Hour is 3600secound
90000:251,2=358
the wheal goes 358 times around to get 89,9 km/h
if your Tyres old,and you are loaded will you have 37x2=74cmx3,14=242,36
242,36cmx358rpm=86564,88km/h
You are saying that the circumfrence of the tire changes enough when loaded as to change the speed reading 
I find that just a little hard to believe
the sant nav is indipendent of the trucks systems and doesnt use wheel circumferance the calulate the vichles speed so sat nav speed will be more accurate than a callibrated tacho though there is a 1second delay in reading
trashwagon:
You are saying that the circumfrence of the tire changes enough when loaded as to change the speed readingI find that just a little hard to believe
Yes. Rubber is compressible. The rubber will be compressed more when you’re fully loaded, and therefore the effective circumference will be smaller.
The speedo reading is affected by loading, tyre wear, tyre pressure, temperature (which affects the pressure inside the tyre as well as the compressibility of the rubber). GPS speed readings are very accurate, as long as the speed is constant. If it’s changing, then it takes a bit of time for the GPS to catch up.
Try comparing the circumference of a brand new tyre with that of a well worn but still legal tyre, on a truck tyre on a brand new tyre, there is a lot more rubber tread than on an older tyre.
So therefore the circumference on an older tyre must be smaller, so it will need more revolutions to cover the same distance than a new tyre.
So your tacho speed will still show 56mph, but your actual road speed will be less.
It’s pretty much the same with aircraft. The indicated air speed(IAS) may show the same IAS, but when flying with the wind, your speed across the ground will be higher than if you’re flying against the wind.
Example: I was flying a Glider, coming in to land, I was much higher than I should have been, too low to sideslip, but taking note of the windsock, I bled the speed off until I was balanced against the wind, my Airspeed indicator said 35Kts, but looking over the side, my across the ground speed was NIL, and I was dropping in altitude slowly.
When I was at the right height to go into land, I just built up forward speed and landed.
I usually speed-check any vehicle I drive on flat ground ASAP, which is easy near me, in north lincs 
get on the M181 or M180 and jobs a good un
makes sure I’m relatively safe. It saw me well looked after when I delivered an ex RAF dog handling van to its new owners and while indicating 15 it was doing 32, and the needle flipped around wildly. most stressful drive that one, carrying 9 points already at the time
you have to bear in mind that tyre-wear has a significant effect and as said above, I’d imagine load would too.
for example, my bike on brand new pirellis (steep profile) will read 69 on the speedo against 70 on GPS. on worn bridgestones (flatter profile) ready for replacement, the speedo reads 70 when GPS shows around 62-63.
so, old tyres on a truck ready for replacement could make 3 or 4 kmph difference as opposed to new ones. a few mm off the circumference revolving hundreds of times per minute adds up.
Agree here. GPS is extremely accurate at constant speeds. My tacho is about 1 kph too fast (which isn’t much). The 4 coaches that we have GPS speeds from 97 to 99 kph even though the set speed is 100 kph ± 5 percent.
el gordo 78:
I usually speed-check any vehicle I drive on flat ground ASAP, which is easy near me, in north lincs
makes sure I’m relatively safe. It saw me well looked after when I delivered an ex RAF dog handling van to its new owners and while indicating 15 it was doing 32, and the needle flipped around wildly. most stressful drive that one, carrying 9 points already at the timeyou have to bear in mind that tyre-wear has a significant effect and as said above, I’d imagine load would too.
for example, my bike on brand new pirellis (steep profile) will read 69 on the speedo against 70 on GPS. on worn bridgestones (flatter profile) ready for replacement, the speedo reads 70 when GPS shows around 62-63.
so, old tyres on a truck ready for replacement could make 3 or 4 kmph difference as opposed to new ones. a few mm off the circumference revolving hundreds of times per minute adds up.
Just as a footnote ElGordo I have noticed of late they have got very keen on speed traps on the 180 lately, never used to see any, now they are almost daily.
As far as I can ascirtain from various websites including the Safety Camera Partnerships the tolerance setting of speed camera’s is 10% plus 2 mph.
If this is true of all camera’s, and please tell me anyone if I am quoting wrongly here, in a Truck on a dual carraigeway whose limit is 50mph unless stated then at 10%+2, being 57 mph, if your on the limiter at 56mph (90kph) then you are unlikely to trigger the unit, particularly if as stated by measured GPS the tacho is invariably reading high by 2-3mph.
I run through the camera’s on the A1 and A14 regularly on the limiter and have had no problem so far, or have I just been lucky that there was no camera or alternately no film in the unit?
Depending on replies I may neec to ease back a bit through the camera’s to be on the safe side.
that’s following ACPO guidelines and is applied in general to cagers and bikes.
An ossifer I know from Derbyshire reckons they often work on 10% flat for commercial drivers as they “should know better” and have more idea, and know their vehicles are supposedly callibrated and more reliable on speed.
depends on the copper.
M180 coppers last summer worked on a blanket 80mph for cars/bikes to prosecute but I understand that is reducing to 78 this summer.
I have no idea what the top speed of my truck is I tried to find out once and lost my nerve when it went off the clock at 80 mph
i thought tachos had to be accurate to + or - 2% given they are tested frequently
…when the new cameras are working on the M42 they will not be as leient as 10% + 2 mph…‘because speedos in new vehicles are more accurate’ thats according to a girl i know who works for the highways agency
As said above, it’s your speedo / tacho mis-reading… The GPS is very accurate…
The law states r=that a speedo must NOT under-read (show 65 when your doin 70) but is allowed upto 10% inaccuracy the other way, so in theory, if your speedo said 66, you could be going as slow as 60…
So far if noticed the following speedo errors…
New scania - 5% out (56 was actually 53ish)
Old SF Ovlovs - suprisingly accurate… well most of them are 5% or less…
My Scooby - 5% as per Scania…
A new Nissan 4x4 we hired t’other day - 8-10%
My old cabby, to which I fitted white dials, which involved removing the needles… !!! Showed 60… GPS said 47… 
Still… Don’t forget, whether your tacho is correct or not, what it shows is what actually goes on the tacho…!!! so don’t think it’s ok to sit at 63 allday cuz the GPS says your really going 56, cuz you’ll be busted come analysis
Luv
Chrisie… 
Ragtop:
As said above, it’s your speedo / tacho mis-reading… The GPS is very accurate…The law states r=that a speedo must NOT under-read (show 65 when your doin 70) but is allowed upto 10% inaccuracy the other way, so in theory, if your speedo said 66, you could be going as slow as 60…
Still… Don’t forget, whether your tacho is correct or not, what it shows is what actually goes on the tacho…!!! so don’t think it’s ok to sit at 63 allday cuz the GPS says your really going 56, cuz you’ll be busted come analysis
Luv
Chrisie…
But didn’t I read on Trucknet somewhere that a tacho on its own couldn’t be used as to wether or not someone had done something wrong, surely you would have to compare that to the true mileage, ie if tacho out, then distance travelled (as measured on tacho) would be proportionatly greater than the actual distance involved, and can be proven?
can anyone enlighten me