MPH out by about 7mph

boyde · Occasional Visitor Joined: Dec 29, 2005 Posts: 9

Not that it is really important, the speed on my tomtom go300 is about 7mph behind. How can I sought this?

mikealder · Posted: Fri Dec 30, 2005 11:37 am Post subject:

What are you using to compare this to, if its the cars speedo then I would sugest the error is the speedo, (or incorrect size tyres etc), Note the GPS speed is at its most accurate on flat straight roads, is the 7MPH error at 70MPH indicated on the speedo? if so 10% is about all I would expect from a car speedo, thats your safety margin for not exceeding the limit - Mike

peterc10 · Posted: Fri Dec 30, 2005 1:24 pm Post subject:

Mike's right - if you are doing a constant speed in a straightish line and on a flatish road it is your speedo not your sat nav that is wrong.

Darren · Posted: Fri Dec 30, 2005 1:35 pm Post subject:

Regulations require that vehicle speedometers are either 100% accurate or can read up to 10% over.

Consequently most speedo's will read somewhere between the two, often towards the 10% fast end.

Your GPS Speed is far more accurate assuming you are travelling at a constant speed.
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Darren Griffin

mossman · Regular Visitor Joined: Dec 21, 2005 Posts: 90

Although I agree with the replies here (after all, the GPS data has to be a much more accurate way of computing speed), for me, it does take a huge leap of faith to start to disregard the speedo and use the GPS speed (even though I know that is is likely to be the more accurate).

Adek · Posted: Sat Dec 31, 2005 3:23 pm Post subject:

a traffic policeman explained to me that speedo's on the car are not all that accurate, as the tread on the tyre actually influences the reading you will get, and hence it will change with time as your tyres change.

There are other parameters which also influence its accuracy, so he explained to me that they re-calibrated their speedo's quite frequently on the patrol cars- I am not sure if this was the cars actual speedo they re-set or some further unit installed by the police.

anyway, the main difference I guess would be on the update speed of the GPS, - the car speedo will react very quckly to changes in the car speed, the gps will be slower I would think.

Adek

Skippy · Posted: Sat Dec 31, 2005 3:32 pm Post subject: Re: MPH out by about 7mph

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