GPS Speed vs Car Speedo

[Madferrit]

Recently indulged in the world of Sat Nav (TomTom 5) and travelled to Crawley last week only to notice the following regarding my speed.

Car: V230
Car speedo: 70mph
GPS speed: 62mph

I was a bit surprised.. but it seemed the faster i went, the larger the gap between my car speed and that being picked up on the SatNav. At 80mph my GPS was telling me i was travelling around 68mph.

I then put the unit into my wifes A160 to see if there was any difference, and again at 70mph my GPS was telling me i was travelling 65mph.

Which one is more acurate?

 

[glojo]

The GPS will be more accurate. Calibrating over a measured distance with a calibrated stop watch will be even better.

It is in a manufacturers interest to have a slightly 'inaccurate' speedo. Just read some posts on these forums of claims about speed! The manufacturers want people to think the car is faster and more economical than it really is.

I believe though it is contrary to numerous regulations to have a speedo that actually dispays a slower speed.

Mercedes-Benz are no better and no worse than any other manufacturer.

 

[prprandall51]

GPS. It is so accurate that some car magazines now use GPS-based timing systems for their 0-60 times.

Car speedos can be up to 10% optimistic and must be at least a little opimistic by law (to be precise, they are not allowed to under-read the speed and no manufacturer can guarantee that their speedo is 100% accurate so they always go a little over). Mine car speedo shows about 4mph over actual (GPS) speed.

Your speedo is very high (but still within the 10% allowed by law). Are your tyre pressures correct? Are you using the factory fit wheel and tyre size?

Philip

 

[Madferrit]

Hi Philip,

I forgot to mention that on both vehicles i am using standard tyres, and tyre sizes.. and the tyre pressure is correct.

I guess i can see the reasoning for the (slight) inaccuracy from a manufacturers point of view.. but it did make me wonder that if i was going to travel at an accurate 70mph, then my speedo would read over 80.. oh well.. at least i now know why my license is safe

 

[No1vic]

GPS speed

I use tom tom mobile 5 too, out of interest I have checked it's speed display against the one in my HGV, which is calibrated every two years, and it is within 1mph. So I would tend to believe the GPS speed display is more accurate.

 

[prprandall51]

Your figures do seem high.

You can also calibrate your speedo very accurately by using the marker posts on the motorway in conjunction with cruise control, as John mentioned above:

Find a quiet motorway. Set cruise at exactly (say) 70mph.

Pass a marker and start a stop watch at that moment (job for a passenger, really). Make a note of the marker number - they are pegged out in 1/10 Km intervals.

Drive a suitable distance (preferably several miles if you can keep the constant speed up). Stop the watch on a marker and make a note of its value then take the figures home to do your sums.

Philip

 

[Ian B Walker]

or stick to 60mph ont the motorway as previously mentioned, time a mile which should take exactly one minute

 

[prprandall51]

or stick to 60mph ont the motorway as previously mentioned, time a mile which should take exactly one minute

But how would you measure the mile out? Not by using the car's trip recorder, which is fed by the same sensor as the speedo and will thus also be innacurate

 

[Madferrit]

Find a quiet motorway.

Around London... are you kidding!?!?!?!?

I've got some motorway cruising to do this weekend.. so might take up yours (or Ians.. which is easier) idea and see how things measure up.

 

[glojo]

Around London... are you kidding!?!?!?!?

I've got some motorway cruising to do this weekend.. so might take up yours (or Ians.. which is easier) idea and see how things measure up.

Every Local Authority will have a 'measured mile' which they usually locate on roads capable of suitable speeds. These sites are used by Police to calibrate speedometers etc. Try asking a 'friendly' Traffic Officer! They are located by poles with black and white strped poles. One band black, one band white etc.

60 mph is the ideal speed for ease of calibration.

John

 

[Ted]

60 mph is the ideal speed for ease of calibration.

John

or 120 mph

 

[Madferrit]

or 120 mph

Can't see the V230 or A160 getting that fast!

 

[McGreggor]

Isn't GPS only accurate to 10square meters for us mere civillians?

 

[PJH]

But how would you measure the mile out? Not by using the car's trip recorder, which is fed by the same sensor as the speedo and will thus also be innacurate

Motorways and most dual carriageways have marker posts every 100 metres either in the central reservation or on the verge.
Time yourself over 1 or 2 kms.

There was this thread last year.

 

[peterchurch]

Around London... are you kidding!?!?!?!?

I've got some motorway cruising to do this weekend.. so might take up yours (or Ians.. which is easier) idea and see how things measure up.

Or you could go watch a late film at bluewater and on the way home you will get the M25 to yourself (aahh those were the days )

 

[prprandall51]

Isn't GPS only accurate to 10square meters for us mere civillians?

It is as accurate for us civilians as it is for the military nowadays. The accuracy is down to the quality of the GPS receiver, signal strength, satellite position and density etc.

However, in measuring speed, GPS is measuring the change in position and so positional innacuracy, which would be reasonably constant over a short distance and time, is "factored out" of the calculation, giving a very accurate speed reading.

I wouldn't be surprised if the skies of Northern Europe are currently teeming with satellites now that Bush is over here and following the dreadful bombings in London.
Philip

 

[marcos]

Surely GPS must suffer a time delay factor as it uses signals from space.

 

[Satch]

Indeed, and a GPS unit measures that very delay in order to work out how far it is from the GPS satellites it is locked on to and thus where it is by a process of three dimesional trilateration.

Every satellite contains an atomic clock, but the GPS receiver uses a bog standard quartz clock, which it constantly resets by looking at incoming signals from four or more satellites, thus allowing it to accurately measure the time delay

 

[Stratman]

Out of interest I fired up my yellow Etrex and used it to check my '97 C180's speedo. I was impesssed.

GPS 70 C180 69.2
GPS 30 C180 29

That'll do for me.

 

[prprandall51]

Out of interest I fired up my yellow Etrex and used it to check my '97 C180's speedo. I was impesssed.

GPS 70 C180 69.2
GPS 30 C180 29

That'll do for me.

Are they those figures the right way round? That's a very accurate speedo reading (nearest .2 mph) and the speedo should always read over, not under.

Whichever way round - that is indeed an incredibly accurate speedo. I over-read by about 5% (74mph at an actual 70).

Philip