SAT NAV SPEED INDICATOR

[laser173]

Hi

Does anyone know if the speed indicated on a sat nav is more accurate than the speedo on a bike or car?

I have noticed when travelling in a car and on a bike that the speed on a sat nav is always lower than the vehicle.


Cheers

Graham

 

[Schtum]

Generally speaking - yes. More precisely - so long as you're travelling at a constant speed, in a straight line, on level ground and have a good satellite fix then yes.......apart from the odd vehicle which has a deadly accurate speedo. The last two Nissans I drove were like this with speedos that matched the GPS exactly.

The NMEA signal updates at a frequency of 1 Hz so you can see that if you're accelerating hard or braking sharply there can be a delay in the GPS unit updating to display your actual road speed.

 

[Womble]

In a straight line at a fixed speed, the GPS value should be more accurate, but it is based on an average reading over about 5-10 seconds, so you will find that it lags behind as you accelerate or decelerate.

The inaccuracies in car/bike speedos are mainly a result of them needing to never under-read because that would make the manufacturers accessories to the crime of speeding. You have to bear in mind that there is quite a difference in the diameter of an overinflated new tyre at a high speed compared to an underinflated worn tyre at a low speed and the fact that the speedo calibration has to sit somewhere in between those values. And that's before all the other inaccuracies due to manufacturing etc. are taken into account. Because the manufacturers play safe, you will generally find that a speedo reads 75-77 mph when the vehicle is actually doing 70.

Of course, the regulations don't apply above 70mph so manufacturers have been known to boost the values above that because it makes the acceleration figures look better. A mate used to work for a UK car company and they apparently added a progressive 10% fudge factor between 70 and 100 mph.

 

[Womble]

Generally speaking - yes. More precisely - so long as you're travelling at a constant speed, in a straight line, on level ground and have a good satellite fix then yes.......apart from the odd vehicle which has a deadly accurate speedo. The last two Nissans I drove were like this with speedos that matched the GPS exactly.

The NMEA signal updates at a frequency of 1 Hz so you can see that if you're accelerating hard or braking sharply there can be a delay in the GPS unit updating to display your actual road speed.

Not only does the signal only update once a second (1Hz), but the signal is fuzzed about with because the American military don't want terrorists using GPS to their advantage. That's why the readings need to be averaged over several seconds.

My KTM pogo has a really accurate speedo, but that's because I can set the wheel circumference on the instrument panel

 

[Schtum]

Not only does the signal only update once a second (1Hz), but the signal is fuzzed about with because the American military don't want terrorists using GPS to their advantage.

One of the things that Clinton did before he left office as US President was to have Selective Availability switched off. Both positioning and speed indication have been more accurate since then -

http://en.wikipedia.org/wiki/Global_Positioning_System#Selective_availability

 

[Womble]

More accurate, but still not fully accurate. I'm never quite sure if SA has completely been turned off, or just turned down a bit...

However, even if the wobbling is a result of some United States 3 letter agency or just inaccuracies in the receivers and calculations, you still need to average the last few readings to that you can be suitably accurate.

 

[Schtum]

More accurate, but still not fully accurate. I'm never quite sure if SA has completely been turned off, or just turned down a bit...

The word is that SA has been completely turned off as it's now possible to degrade and completely block the satellite signal in selected areas.

However, even if the wobbling is a result of some United States 3 letter agency or just inaccuracies in the receivers and calculations, you still need to average the last few readings to that you can be suitably accurate.

That's not my experience comparing the speedos in the last two Nissans I drove - a Murano in the Western USA and a Note in Lanzarote - with two different Garmin GPS units. The speedos and GPS units matched each other exactly.

 

[Womble]

The word is that SA has been completely turned off as it's now possible to degrade and completely block the satellite signal in selected areas.



That's not my experience comparing the speedos in the last two Nissans I drove - a Murano in the Western USA and a Note in Lanzarote - with two different Garmin GPS units. The speedos and GPS units matched each other exactly.

OK, so it looks like the inaccuracies are down to the GPS signal and the calculations, not a wobbly signal, which is good news.

No matter what, you still need to be careful about the whole averaging thing - my Zumo and my GS speedo can often differ by 10 mph during heavy acceleration or deceleration. All I'm saying is that people need to pay attention and not assume that a GPS is more accurate, just because it has blinkenlights and flashenstuff.

Also, I certainly wouldn't want to replace a vehicle speedo with a GPS, but I always use a GPS to calibrate the vehicle speedo.

 

[Neil W]

We've tried a number of Garmins and Tom Toms at work against various 'CALIBRATED' speedometers and they have all been within 1 mph at an indicated 70mph (ie a damned sight better than most car speedos).

 

[IJGser]

10% average

Most cars seem to be about 10% out e.g showing 77 doing 70

Ever wondered why in those 50 mph average speed camera road works HGV's seem to or want to go faster than cars? Answer is they have calibrated speedos and the cars are doing MPH.

 

[IJGser]

oopps

Cars are doing 45

 

[Giles]

Your Sat nav will be Amazingly accurate!

The Police traffic cars that are all equipped with calibrated speed devices (ie, a device in the dashboard, not just a callibrated speedo..), are ruthlessly accurate. They are checked every day and re-callibrated every 28.

If you shove a sat nav in the car, (well lets be specific here, I have used my own zumo 550 in the cars before....) the speed on the sat nav will mirror the display on the 'Police Pilot' exactly. From walking pace to 150, it is absolutely spot on.

 

[Sgt Bilco]

Ever wondered why in those 50 mph average speed camera road works HGV's seem to or want to go faster than cars? Answer is they have calibrated speedos and the cars are doing MPH.

Exactly I hate the average speed cams on the motorways because most people are at 45mph when the speedo's are at 50.

 

[Surrey Yeoman]

Speed display on my TomTom Rider 2 reacts almost instananeously to variations in speed. Haven't ridden it through a laser check yet, but I reckon it's spot on.

 

[LoggyDriver]

Most cars seem to be about 10% out e.g showing 77 doing 70

Ever wondered why in those 50 mph average speed camera road works HGV's seem to or want to go faster than cars? Answer is they have calibrated speedos and the cars are doing MPH.

Ah, someone that understands at last..

I'm a truck driver and it does my head in when I'm in roadworks and there are cars in the middle lane doing 40-45mph. Unlike a car we can't pull out to overtake in the third lane. HGV's have very accurate speedo's (calibrated) compaired to cars. I personally will sit at 49mph through a 50mph roadworks. Being held up doing 40 is VERY frustrating.

 

[bose]

sorry this isnt true

Ah, someone that understands at last..

I'm a truck driver and it does my head in when I'm in roadworks and there are cars in the middle lane doing 40-45mph. Unlike a car we can't pull out to overtake in the third lane. HGV's have very accurate speedo's (calibrated) compaired to cars. I personally will sit at 49mph through a 50mph roadworks. Being held up doing 40 is VERY frustrating.

I to drive a truck an have a digi tacho but run it along my zumo 550 and at 80kmh im doing 47mph so theyre not as calibrated as you may think

 

[LoggyDriver]

I to drive a truck an have a digi tacho but run it along my zumo 550 and at 80kmh im doing 47mph so theyre not as calibrated as you may think

When was yours calibrated, what meat did you have on the tyres at the time and how much have they worn? My trucks tacho is spot on to what my GO530 says I'm doing.