Speedometer or GPS

pistolpetes11

I know this may be a stupid question but the last few times that I have been using my TOM TOM GPS , I noticed that it always seems to display a few KM/H less than the spedo and it got me thinking which is more accurate ?

Say I have cruise control set at 120 KM/H the GPS always shows 116 KM/H

Same with lower speeds.

I drive a Nissan Pathfinder .

FGR

Most car speedos are overcalibrated - I believe most motor companies do it as a precaution as the threshold is high enough to prevent undercalibration which could possibly happen to the speedo of the course of a car's life. If that happened it could put a motorist in a dangerous situation when they don't realise they're going faster than they think.

Your GPS is most accurate in this case.

fabmorretti

Those handheld GPS units give +/- 10/20m accuracy dependent on a lot of factors e.g. no. of satellites, weather, the relief of the land etc.

Don't know if you could fully rely on them for an accurate value.

Hooch

fabmorretti wrote: »

Those handheld GPS units give +/- 10/20m accuracy dependent on a lot of factors e.g. no. of satellites, weather, the relief of the land etc.

Don't know if you could fully rely on them for an accurate value.

We tested this some time back in an ANPR car checking out the calibration. The ANPR system has both calibrated and GPS reading. We added a Garmin sat nav just to see. Drove down the road and pinged it with a speed camera.

Calibrated ANPR, GPS ANPR and Sat nav were all the same. The mondeos speedo was +6 kmph off at 100 kmph.

In short.....GPS is most accurate.

Alkers

GPS will only give speed over level ground. i.e. if you're going up or downhill gps would read your speed as lower than the actual speed your doing.

corktina

all my cars (6 of them) are around 10% different to the GPS speedo I have. I tend to bear this in mind.

I agree hills would make a difference to the reading but this would only be very slight and only would be noticeable I feel on a steep hill over a long distance, there arent many of them in Ireland.The angle of the planets surface in relation to a satelite would only vary very slightly indeed

rebel.ranter

Just on the accuracy of the positioning +/- 10m comment. The accuracy of your exact position can change depending on how accurate the American military want it to be. They switched the whole thing off during the Iraq war for a spell!
It should not really matter how accurate a GPS location is to get a speed reading. Speed is a measure of distance traveled in a set period of time. If your originating location is out by 10m then so is your new location, the key is your distance travelled is the same & obviously time is accurate so there is no reason not to believe that for a flat stretch of road that you GPS would give an inaccurate speed.

mullingar

nice guy always wrote: »

Drove down the road and pinged it with a speed camera.

What a job, testing speed cameras;). Anybody got a Veyron I could borrow?? :D

101sean

Wherever there are speed cameras in the UK, especially specs types, general traffic speeds are around 5mph less tha the limit because of speedo inaccuracy. Surprisingly, the speedos in both my Land Rovers are spot on with the GPS.

Selective Availability was turned off 10 years ago although the US could use it again, partly why the EU is launching it's own system. Interesting to read that in Gulf 1, the US had to turn it off as soldiers were buying civvy units due to supply shortages (nothing new there then :rolleyes:) and it was causing problems.
http://www.pocketgpsworld.com/GPS-Selective-Availability-lifted-10-years-ago-9500.php

keithclancy

Simona1986 wrote: »

GPS will only give speed over level ground. i.e. if you're going up or downhill gps would read your speed as lower than the actual speed your doing.

Maybe in Norway, Ireland doesn't have the hills for it

IMO, I would expect a SatNav to be accurance, having had a Tomtom for the last 3 years i've always found the deviation between the Speedo and the SatNav the same.

robtri

neither the speedo or the gps are acurate.... both are flawed by design.

Speedo, as above is over calibrated by design..

GPS, is not very good at acurate speed readings going up or down inclines.

keithclancy

robtri wrote: »

neither the speedo or the gps are acurate.... both are flawed by design.

Speedo, as above is over calibrated by design..

GPS, is not very good at acurate speed readings going up or down inclines.

Thats the point though, i've never seen that and the deviation is the same between the fixed speedo and the GPS, the Speed in the car would not be affected by Inclines!

In my experience it has less accuracy when in a Town center with high buildings and sometimes shows your moving even when your standing still.

mullingar

GPS receivers calculate your 3D position (Latitude, longitude AND altitude) and is updated [normally] ever second.

These DO calculate your true 3D speed, even if you are going up a 45 degree hill, as they use a standard built in mathematical equation to calculate your 3D speed based on your 3D position before and 3D position now.

corktina

triganomatry (and spelling) isnt my strong point, but it does seem to me that given two axis are VERY long and the third is VERY short, the differences in the angles at the two points on the surface measured on an incline or a flat road will be so small as to be negligible.

keithclancy

corktina wrote: »

triganomatry (and spelling) isnt my strong point, but it does seem to me that given two axis are VERY long and the third is VERY short, the differences in the angles at the two points on the surface measured on an incline or a flat road will be so small as to be negligible.

That would really depend on how many points of reference you have to calculate from.

unkel

ForegoneReality wrote: »

I believe most motor companies do it as a precaution as the threshold is high enough to prevent undercalibration which could possibly happen to the speedo of the course of a car's life. If that happened it could put a motorist in a dangerous situation when they don't realise they're going faster than they think.

It's actually a legal requirement (European directive):

the speed on the speedo can be anywhere between the actual speed and the actual speed + 10% + 4km/h

So if your actual speed is 160km/h, this can be anything on the speedo between 160km/h and 180km/h

Atomic Pineapple

unkel wrote: »

It's actually a legal requirement (European directive):

the speed on the speedo can be anywhere between the actual speed and the actual speed + 10% + 4km/h

So if your actual speed is 160km/h, this can be anything on the speedo between 160km/h and 180km/h

Yep, my mothers brand new Fiesta is only doing 115km/h when the needle is dead on 120km/h

A GPS unit will almost always be more accurate than a mechanical speedometer reading

Cookie_Monster

how much would changing the size of the wheels affect the speedo reading?

Assuming the tyre depth is the same would going from 16" to 17" make a noticeable difference?

I'm presuming the speed reading has to be taken from either the axle or wheel, can't think of any other way to do it, so changing wheel size will cause the axle to rotate at different speeds at the same "actual" speed won't it.

mailforkev

Cookie_Monster wrote: »

how much would changing the size of the wheels affect the speedo reading?

Assuming the tyre depth is the same would going from 16" to 17" make a noticeable difference?

It must make a slight difference. I know that the model of my own car that has 16" wheels as standard requires a software update to recalibrate the speedo sensors if you upgrade to 18s but not if you only go up to the 17s.

rebel.ranter

When you are changing the wheel size, say from 16' to 17' you normally reduce the profile of the tyre at the same time to keep the overall rolling radius the same. More wheel less tyre. The difference in the rolling radius is then reduced to an absolute minimum.
Sure even a new tyre will give a different rolling radius to a barely legal tyre, OK that is not a huge difference either.
However if they are re-calibrating speedos on some cars when you change wheel then that can only be a good thing.

mullingar

Here is an online calculator to tell you by what % your new wheel size is affecting your speedo. (about 2/3 way down)

http://www.carbibles.com/tyre_bible_pg4.html

Joe 90

unkel wrote: »

It's actually a legal requirement (European directive):

the speed on the speedo can be anywhere between the actual speed and the actual speed + 10% + 4km/h

So if your actual speed is 160km/h, this can be anything on the speedo between 160km/h and 180km/h

It used to be the UK law that the speedo should not under read and could over read by up to 10%, I always assumed that Irish law was the same as it generally was pretty much the same. I was not aware of the EU version of 10% + 4 klicks.

It used to be said that the car manufacturer would in fact aim for the +10% as the target accuracy so that the owner of his car could boast of the performance. It was also said that the milage recorded was also calibrated to over read by 10% so that the proud owner would be happier with the fuel consumption. With the milage over reading the car would be service more often so the dealer was happier too. Everyone happy, what a wonderful world.