How accurate are Sat Nav speed clocks

[stormforce]

Last week when driving to Tony & Linda's i had 31x1050x15's on the truck and the satnav was reading 5mph less than the speedo on the truck.So doing 70mph on the speedo the SatNav reads 65mph.

I have just taken the truck out and when the speedo reads 70mph the SatNav reads 69mph and
at 60mph on the speedo the SatNav reads 59mph.
When the speedo reads 30mph the satnav reads27/28mph and i am now running 33x1250x15's.
All i really need to know is do these speeds sound about right or will i be better off doing a speed test with another motor
Thanks all

[nero279]

the sat-nav should be accurate in a straight line as long as it's not running slowly/short on memory. If the satnav is accurate with your position at junctions etc and not 'lagging' then the sat nav is probably as accurate as your gonna get.

I think i'm right in saying car manufacturers have a certain allowance on new cars that the speedo can read over, but not under. So generally car speedo's on the factory tyre size will read a few mph over your actual speed anyway.

[joker??]

sat nav should show a truer reading than your speedo rich

[John McC]

More sats equals better accuracy and as long as its in a straight line on a flat road should be about as good as you'll get. As to the speedo most cars are about 10% high so bigger tyre the nearer actual speed will be on the speedo will be right

[plumb bob]

With 31"x10.5x15" on mine, my sat nav gives the exact same reading as my speedo.

[stormforce]

Thanks guys.This is what i thought but just thought its best to double check

[gwh200]

Last week when driving to Tony & Linda's i had 31x1050x15's on the truck and the satnav was reading 5mph less than the speedo on the truck.So doing 70mph on the speedo the SatNav reads 65mph.

I have just taken the truck out and when the speedo reads 70mph the SatNav reads 69mph and
at 60mph on the speedo the SatNav reads 59mph.
When the speedo reads 30mph the satnav reads27/28mph and i am now running 33x1250x15's.
All i really need to know is do these speeds sound about right or will i be better off doing a speed test with another motor
Thanks all

dude,hit your sat nav with a big stick!!!

its not that accurate only within a few metres or so.you need one of apa
ches radar thingys to get a real accurate reading.as you have fitted bigger tyres, you may find that a your speed increases so does the innacuracy of your speedo,because the tyres are going around"less" your speed (say an indicated 100kmph) will say the obvious you are travelling at 100 kim per hour or 1.6km per minute or 0.027 metres per second.that 0.027 metres may be a bit of the extra circumference of your tyre,so in not making a full revolutin with respect to your speedo reading

its to small really to make much difference,other than to say as your speed increases with respect to distance travelled on calibrayed tyres,the reading you get will be slightly optomistic.
look on the bright side you shouldn't get a speeding ticket

[da SLUG man]

my tom tom usually matches the speedo upto 30mph and then reads lower than the speedo... 70 on the speedo is only 65 on satnav

[BUSHWHACKER]

my tom tom usually matches the speedo upto 30mph and then reads lower than the speedo... 70 on the speedo is only 65 on satnav

Same here.

[da SLUG man]

went out for a walk with the dog while tracking on satnav, when i downloaded the results, it showed my fastest speed as 22mph and had quite a few squiggles where it had lost signal etc.

[Apache]

dude,hit your sat nav with a big stick!!!

its not that accurate only within a few metres or so.

Navigation-wise you're right. However, speed wise they are VERY accurate when travelling in a straight line at a constant speed. Much more accurate than a drivetrain driven speedo.

[onlynik]

There is also the 600ms delay between the send and recieve on the GPS unit.

[Apache]

There is also the 600ms delay between the send and recieve on the GPS unit.

Not sure of the relevance of the time it takes for the signal from the sat to hit the rx...

[onlynik]

Latency is an issue unless its coded into the device to compensate.

[Apache]

It has to be to a certain extent in order for any kind of sensible accuracy for navigation.

The inaccuracy latency produces is in the noise when it comes to velocity calculation.