Safe Speed Forums

I have recently purchased a navigation system, which has an option to show your current speed, however it seems that the speed shown via the gps system is some 4 mph lower that my car's speed reading...

now i am not sure if this is correct, and i am assuming that my car's reading is not as acurate as the gps one....

can anyone help me out?

Car speedos tend to over-read - I believe that the tolerance allowed is a few percent, but only in the "upwards" direction, so it's reasonable to assume that the GPS is correct.

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"Politicians are the same the world over... We build bridges where there aren't any rivers." - Nikita Kruschev

AIUI speedos are allowed to be up to 10% inaccurate, but that they must only over read. Sounds like this is what's happening to you. You may well find that it's not always exactly 4mph difference either. Could be that there's a speed where your speedo is virtualy spot on by the GPS, but below and above this accuracy falls away. One caveat with the 10% bit... it effectively applies only when the car leaves the factory. An older car could potentially be off by more than 10% sometimes.

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Make everything as simple as possible, but not simpler - Einstein

Also remember that with the GPS that it will be inaccurate during heavy acceleration and breaking and also if your going up/down steep hills.

I'm currently having to use my GPS as speedo is broken (reading about half speed - local Alfa dealership want £600+ for a new speedo - they are going to be waiting a long time as thats about half the value of the car).

Thanks guys,

Just to explain further I have a new car, and i have noticed the speed difference using cruse control. but now it makes sence.....

i was gong 70 mph and gps recored 66 mhp, on a motorway.....

No No, you were going 66 mph but the speedo read 70 mph

Just hope you aren't stopped, as doesn't this technically make your vehicle unroadworthy? Although I suppose you could argue that the GPS is your speedo..

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-mike[F]
Caught in the rush of the crowd, lost in a wall of sound..

I have a friend who ran his car with a bicycle computer as its speedo for a few months

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Chris
www.chrishillcoat.net

Won't most faults end up with the needle pointing at zero all the time? And if the car is parked up mid-trafplod lecture as far as the BiBs concerned it's going to be reading right . I think this is one of those that is normally very difficult for the police to enforce. I don't know, but I think MOT testers are supposed to fail for a non-moving speedo needle. I've never asked one but I noticed on my car's last test that "speedo" appears at the bottom of the check list. I'm certain they're not checking speedo accuracy in the MOT, so all I can think that bit means is that they're expecting to see the thing move when the car is on the rolling road and will fail if it stays still. Anyone know a tester who can shed light on this?

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Make everything as simple as possible, but not simpler - Einstein

I know that changeing the tires can affect the speed reading, making sure they are correctly inflated may affect it a bit too.

I choose to trust GPS over the car's speedo however you must remember that GPS is an average of the past few seconds/miliseconds. So cruise control is good for this test. It's also good for overtaking police cars on a motorway

Tires wearing down definatly affects the reading. For example if you notice you're doing 82 past a talivan and think "well that's only ~74 last time I checked with GPS" then check with GPS the next day and find that the speedo is pretty damned accurate at 82 since you changed the front tire....

(I don't know how they didn't catch me! Must have been the randomness of the laser working in my favour.)

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Andrew.

To be pedantic.

10%+6.25mph at all speeds over 25mph

And yes, only when the car is new.

Mine reads within 2mph up to 70mph but the Mondeo we have at work reads 60 at 55 and you would have to have over 85mph on the clock to be into the prosecution threshold for a 70mph zone.

It should be said that GPS is as accurate as you will ever need for practical purposes as long as you have a clear signal and aren't changing speed.

I have a TomTom setup on my PDA and a Garmin, and they both read exactly the same, and are spot on when calibrated against furlong posts on the motorway.

The radar on my tractor goes bonkers under 1km/hr, it tells me all sorts of lies, so I can't measure distance on it that slow. It doesn't give me a number to put in to the dash set up to just take speed off the radar un like the (insert spitting noise) blues ones and have to change the calibration number when I change wheels.