How accurate is Sat Nav speed readings

Treadway1

I think the answer is that whilst GPS may well be more accurate, the speedo will always be more reliable. Even with a slight discrepancy allowing for new/old tyres, and manafacture design, the speedo is a constant, whilst the GPS is liable to suddenly lose signal, and therefore integrity.

pinkgirl2009

if you get a ticket ask to go on a speeding course it will cost you your fine and lose the points. i was on last week. they told me you drive at 30mph you are doing 28mph allcars . this is what i was told. but it will save you in insurance for the 3 points. hope this helps. pop a note in ans the fine money and ask for the couses

vikingaero

Treadway1 wrote: »

I think the answer is that whilst GPS may well be more accurate, the speedo will always be more reliable. Even with a slight discrepancy allowing for new/old tyres, and manafacture design, the speedo is a constant, whilst the GPS is liable to suddenly lose signal, and therefore integrity.

If you follow an old car - a Ford Escort for example - through a 30 mph zone, you'll find that the driver will think he's doing 30mph when in reality he'll be doing 23 or 25 mph. This is due to the cable driven speedo which is pretty inaccurate at low speeds.

On more modern cars fitted with ABS, the ABS sensors are used to calculate speed and a speed pulse is sent up to the speedo via the ECU/Canbus. Because the ABS sensors are in fixed positions (including an allowance for tyre wear) each rotation can measure speed more accurately - so an ABS equipped car doing an indicated 30 mph is more likely to be travelling at 28 mph.

Alexei

Treadway1 wrote: »

I think the answer is that whilst GPS may well be more accurate, the speedo will always be more reliable. Even with a slight discrepancy allowing for new/old tyres, and manafacture design, the speedo is a constant, whilst the GPS is liable to suddenly lose signal, and therefore integrity.

Yes, a typical speedo is reliable - reliably too high by up to 10%, as others have said. If your GPS (when going at a steady speed, with good signal, on level ground) tells you you're going 70 when your speedo says 77, you don't need your GPS to tell you the speed any more - any time you see 77 on your speedo (in the same car), you'll know that you're actually going at 70.

Treadway1

Alexei wrote: »

Yes, a typical speedo is reliable - reliably too high by up to 10%, as others have said. If your GPS (when going at a steady speed, with good signal, on level ground) tells you you're going 70 when your speedo says 77, you don't need your GPS to tell you the speed any more - any time you see 77 on your speedo (in the same car), you'll know that you're actually going at 70.

I see what youre saying and agree, but I bet you wouldnt dare to cruise past a Police car thats doing indicated 70 on a motorway, and then try to give that excuse..... :rotfl:

jamesp31

im quite interested by this post. I had an RAC sat nav for a few years where I never noticed a difference between the speed on the speedo and the satnav. I then used my dads Garmin for a trip to London and did notice that the speed was pretty similar between my speedo and the satnav. I have now just bought a Tomtom XL, and on the first time using it noticed a difference in speed by about 5 miles per hour, which straight away made me think, 'this sat nav is not as good as ones ive used'. But now reading this it seems that its quite normal. Anyone else got a tomtom XL?

Armengar

I thought manufacturers certified speedos according to the largest wheelsize for the range? Therefore smaller wheels will overread whereas larger wheels will be spot on?

Nilrem

IIRC as has already been said the Speedo on the car is allowed to read up to 10% above the actual speed the car is doing, due to differences in the likes of tyre wear.
Some car's speedos are much more accurate than others.

A sat nav (at least a decent one with a lock on more than the bare minimum of satellites*), will give a more accurate reading on the straight - however it's a reading from the past X seconds since it managed to get an update from enough satellites, so you could be going faster or slower at that particular moment than it's last reading.

So the speedo is the immediate reading, whilst the satnav might be the more accurate one, however the police probably won't be too happy with the excuse "my sat nav said i was at the right speed"

Personally I tend to go by the speedo


*My sisters RAC one has trouble getting a good lock on the bare minimum of satellites (and often shows me running in the middle of a field), my tomtom one v3 usually locks on to 6-9 at any one time.