What is more accurate at showing your speed, your tacho or GPS ?
Tacho seeing as it has to be calibrated yearly to be as accurate as possible. Sat nav are not as accurate as people often believe I think due to sat nav can’t take into account your elevation or something like that I read somewhere. A GPS would be much more accurate if the test was done on the same level all the time.
Volvo use GPS for their I see system and dynafleet, surely this has to be very accurate.
Warning, this post contains information gleaned from my memory and MAY contain errors.
The sat nav works by using satellites to triangulate your position, effectively working out how far from each satellite you are, and overlaying that information onto a stored map. The more satellites used, the more accurate the position.
From this point, working out speed is relatively easy, simply measure the time taken to move from one known location to another. Bingo, speed. The smaller the distance between the two measuring points, the more accurate the speed calculation is.
Because the system calculates your location relative to the satellites, any incline should be irrelevant for speed calculations, not to mention that the time between measurements are so small that unless you’re falling off a cliff, changes in height are insignificant.
The tacho takes its speed from the rotation of the wheels. If it knows the size of them, it simply works out how many times it rotates in a set time to work out speed. The problem is, how round are the tyres and has the tread worn down since it was calibrated.
IMO sat nav speed would be more reliable.
Found this.
Many GPS devices display your current speed, based on a simple calculation of how much distance you are covering in a given time period. However, drivers are often confused with these readings since it generally varies significantly from the vehicle’s speedometer.
So which speedometer is more accurate? Which one should be used to determine if you are driving within the posted speed limits? How does it relate to using speeding alerts in your telematics program?
Speeding is widely regarded as a significant contributor to fatal road accidents, and fleet owners find GPS-based speed calculations very useful in maintaining fleet safety. It’s important then that these speed calculations can be trusted to avoid drivers being falsely accused of speeding, as well as providing a reliable guide to determining a legally-safe driving speed.
How does a GPS device calculate speed?
Using GPS, a device is able to calculate a lot of information about a moving object. Using even basic time and location data, a GPS unit can quickly calculate the relative speed of the object, based on how much distance it covered in a given time.
GPS devices are positional speedometers, based on how far the device has moved since the last measurement. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. It should also be noted that the speed reading is normalized, and is not an instant speed.
Speeds are updated at short intervals to maintain accuracy at all times. It uses frequent calculations to determine the vehicle’s speed. For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH.
Why does GPS speed differ from the vehicle’s own speedometer?
GPS speed calculations are more accurate than a vehicle’s speedometer since it is not affected by the same inaccuracies, including the vehicle’s wheel size or drive ratios. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal.
Vehicle speedometers also require calibration to maintain perfect accuracy as general wear and tear, changing wheel sizes, and the manufacturers own ‘erring on the side of caution’ and setting the speedometer to read higher than the actual speed, all contribute to inaccuracies.
Generally most manufacturers claim a 0.1MPH tolerance with speed readings, however will go as far to say a 0.5MPH inaccuracy is possible, to make allowances for any satellite signal variations beyond their control.
Which speedometer should a driver use?
It really comes down to personal preference since both gauges are reliable for determining a safe, legal traveling speed. In most cases the vehicle’s speedometer will show a higher speed due to the aforementioned inaccuracies but this at least ensures the driver stays lower than the posted speed limit.
And while logged GPS data has been used to overturn speeding tickets, the cautious driver, who is keen to avoid infringements, fines, trouble from their employer or the possibility of an accident, will tend to err on the side of driving slower rather than pushing it too close to the limit.
Using GPS to record company speeding incidents
Any business that has company vehicles available for employee use is no doubt keen to avoid dangerous speeding, both from a public liability point of view, the possibility of accidents as well as infringement notices or demerit points. Using fleet management software such as Telogis Fleet, fleet owners are easily able to specify speeding notifications that can alert managers to speeding incidents in real time, or provide regular reports of this sort of activity.
Speeding alerts can be setup to record either speeds in excess of the posted speed limit, or simply if a certain speed is reached (ideal for vehicles that become unsafe above certain speeds e.g. forklifts).
How closely the speeding alerts are monitored can be set as well, such as how long the vehicle is speeding before the alert is triggered. This can be used to avoid false positive reports due to overtaking or temporary distraction.
Since employers know that speeds calculated by GPS devices are not only more accurate than a vehicle’s speedometer but also lower, it gives employees no excuse to be caught driving at unsafe speeds. Through a combination of regular reporting and driver coaching, fleet owners can make sure they are doing everything in their power to improve the safety of their staff and other road users.
I would say gps is more accurate.
All our trucks say 89kph on the tacho when running on the limiter. When driving the older 14 plate trucks, the satnav says 56mph. The newer trucks from 15 plates onwards all run slower, normally 53/54mph and the satnav backs this up.
Our 5 new fh4’s are the same saying 56 on the tacho but only showing 54 on GPS.
The following info came from an inspector on the local roads policing unit:
" most vehicle speedos are 2-3 mph under the the actual speed they are doing so if your speedo says 30 your more likely to be doing 27-28mph "
Captain Caveman 76:
Warning, this post contains information gleaned from my memory and MAY contain errors.The sat nav works by using satellites to triangulate your position, effectively working out how far from each satellite you are, and overlaying that information onto a stored map. The more satellites used, the more accurate the position.
From this point, working out speed is relatively easy, simply measure the time taken to move from one known location to another. Bingo, speed. The smaller the distance between the two measuring points, the more accurate the speed calculation is.
Because the system calculates your location relative to the satellites, any incline should be irrelevant for speed calculations, not to mention that the time between measurements are so small that unless you’re falling off a cliff, changes in height are insignificant.
The tacho takes its speed from the rotation of the wheels. If it knows the size of them, it simply works out how many times it rotates in a set time to work out speed. The problem is, how round are the tyres and has the tread worn down since it was calibrated.
IMO sat nav speed would be more reliable.
The rotation of the wheels is to measure the wheels which is where you get the L Factor captain ,the K W is pulses from the cogbox
You generally have 2 type of roller set up ,1 set use speed ,run the vehicle at 50kph to get the K W factor ,and the other set use distance ,so you can drive at 5kph and it measure it over a km…Our equiptments calibration is checked 6 monthly ,rollers are measured and the equiptments K factor checked
Sat navs ,may be accurate ,but they are not VOSA approved ,so they don’t count ,no matter if it says you are doing less than the trucks tacho
speedometer everytime
best example i can give is when i drove a man wagon and drag,after hardly keeping up with tesco trucks despite my tacho reading 56 i put my gps on and was barely reading just over 50 mph…rear tyres were changed for the mot and suddenly it was hitting 56 on both speedo and gps…2 weeks later we were all turned down to 52mph to save fuel!!
a guy i used to know had tacho tyres…he would fit his just legal recuts for calibration then refit new tyres and voila…nearly 60mph.
How is it then that you can have two identical trucks, both show 90kph on the speedo yet one is faster than the other? We had this at my last place. We had 3 Daf CF’s. Two would do 56 on the button yet the 3rd would do 58 according to the sat-nav. It would leave almost everything for dead. All the time driving it I was only ever overtaken twice. Once by a Tang and the other by a subbie pulling a Yodle trailer.
I always use my satnav for current speed information. Some trucks show the same speed as the satnav some don’t. Interestingly the ones that show you are doing 56 mph and satnav shows 59mph pass almost every other truck on the road. The trucks that show you are doing 56mph when the satnav says 52mph are passed by most other trucks on the road. I have drawn my own conclusions. 
xamtex:
best example i can give is when i drove a man wagon and drag,after hardly keeping up with tesco trucks despite my tacho reading 56 i put my gps on and was barely reading just over 50 mph…rear tyres were changed for the mot and suddenly it was hitting 56 on both speedo and gps…2 weeks later we were all turned down to 52mph to save fuel!!
a guy i used to know had tacho tyres…he would fit his just legal recuts for calibration then refit new tyres and voila…nearly 60mph.
Measurement of the tyre circumfrance is part of the calibration ,so if new tyres are fitted after calibration then it will alter the speed slightly ,hence a lot of owner drivers put worn /legal tyres on when calibration is being done …Then take it away to their yard and refit their new tyres…Due to various factors /tolerances in the vehicle ,then speed will alter slightly Hence the 2 yearly calibratuion …Again no matter how good satnavs are ,we are stuck with rollers and 2 yearlys …Also as some one will no doubt mention its slower after a calibration etc ,all figure are calculated automatically with the computer ,the tech has no say or input in what it spits out …I can run a vehicle up 3 times in a row and will get different figure each time .though they wouldn’t make a noticeable difference …So the system isn’t perfect but its what we have and it is what we have to use …Tyre pressures make a difference
Radar19:
How is it then that you can have two identical trucks, both show 90kph on the speedo yet one is faster than the other? We had this at my last place. We had 3 Daf CF’s. Two would do 56 on the button yet the 3rd would do 58 according to the sat-nav. It would leave almost everything for dead. All the time driving it I was only ever overtaken twice. Once by a Tang and the other by a subbie pulling a Yodle trailer.
Because tyres vary in size, the speedometer only measures the speed the wheel is spinning, the bigger the tyre the fast you will go for the same 56 shown!
tyres wear and become smaller and also different brands, even different batches of the same brand can vary in size a little.
One way to discover how accurate the tacho is - measure with the speedometer exactly how far you have driven on a particular route, (make it at least 100 mile) then use some mapping software to measure the true distance, you can now compare the two figures and get a percentage of its accuracy.
if for instance the tacho shows 120 mile but the mapping software shows only 115. then your speedometer is over reading by 4.3%. Therefor when it shows 56 you are probably only doing 53.7mph
Bluey Circles:
Radar19:
How is it then that you can have two identical trucks, both show 90kph on the speedo yet one is faster than the other? We had this at my last place. We had 3 Daf CF’s. Two would do 56 on the button yet the 3rd would do 58 according to the sat-nav. It would leave almost everything for dead. All the time driving it I was only ever overtaken twice. Once by a Tang and the other by a subbie pulling a Yodle trailer.Because tyres vary in size, the speedometer only measures the speed the wheel is spinning, the bigger the tyre the fast you will go for the same 56 shown!
tyres wear and become smaller and also different brands, even different batches of the same brand can vary in size a little.One way to discover how accurate the tacho is - measure with the speedometer exactly how far you have driven on a particular route, (make it at least 100 mile) then use some mapping software to measure the true distance, you can now compare the two figures and get a percentage of its accuracy.
if for instance the tacho shows 120 mile but the mapping software shows only 115. then your speedometer is over reading by 4.3%. Therefor when it shows 56 you are probably only doing 53.7mph
This could be a ploy by manufacturers to manipulate fuel economy figures. Tacho says you have done 400 miles, but in reality it’s only 370 miles. It now looks like you’ve gone further on a tank of fuel than you really have, making the truck look like it’s giving a better mpg than it really is.
Would fitting different circumference tyres affect the speedo ?
mrcreosote90:
Would fitting different circumference tyres affect the speedo ?
yes it just converts the rpm of the propshaft into a speed, change the diameter of the tyre and you will go faster or slower for the same speed on your speedometer.
But of course this will all change with the new generation of tacho’s (march 2018 ?) they have to have two separate inputs for speed (presumably gps & propshaft ?) and an error will be created if there is a significant difference - so no more fitting worn out remoulds for calibration and no more magnets)
And since the police & vosa will be able to read these new tachos remotely there will be no more hoping you don’t get pulled - who knows the tacho may even email the lovely beverly and tell her you have been a naughty boy.
Bluey Circles:
mrcreosote90:
Would fitting different circumference tyres affect the speedo ?yes it just converts the rpm of the propshaft into a speed, change the diameter of the tyre and you will go faster or slower for the same speed on your speedometer.
But of course this will all change with the new generation of tacho’s (march 2018 ?) they have to have two separate inputs for speed (presumably gps & propshaft ?) and an error will be created if there is a significant difference - so no more fitting worn out remoulds for calibration and no more magnets)
And since the police & vosa will be able to read these new tachos remotely there will be no more hoping you don’t get pulled - who knows the tacho may even email the lovely beverly and tell her you have been a naughty boy.
Bluey current tachos use the ebs as a second speed source ,so t will throw up a fault if there is a speed conflict
norb:
Bluey current tachos use the ebs as a second speed source ,so t will throw up a fault if there is a speed conflict
I know a lot of speed sensors now use the abs, but I had thought it was an instead-of rather than an as-well-as.
the documentation for the next generation tachos does make a specific point about dual inputs for speed, so I guess it is just making this law rather than just good practise.