Displayed speed - how accurate?

Nezza42 · Occasional Visitor Joined: Jul 09, 2005 Posts: 15

My car is a Japanes import that has had the speedo face converted from kmh to mph, trouble is, it isn't very accurate, over-reading by upto 10mph or more!

I was wondering how accurate the speed display on my Go 500 is likely to be, any ideas?????

Cheers
Nez

sgtbanjo · Occasional Visitor Joined: May 29, 2005 Posts: 27

I believe it's accurate to 0.1 mph!

Sarge

rayflan · Frequent Visitor Joined: Apr 06, 2005 Posts: 277

My go normally shows a few mph below my speedo ie go 66 speedo 70 but remember the gos info will be a few seconds slow.
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Andy_P · Posted: Tue Jul 12, 2005 1:19 am Post subject:

It's been said here before that "If you are driving a UK vehicle you will find that your car speedometer will report a higher speed than the GPS speed."
Something like +10%, minus 0% was mentioned.
So its very loikely that the Go is the accurate one.

Nezza42 · Occasional Visitor Joined: Jul 09, 2005 Posts: 15

I agree, as Andy says, speedo's in the UK do over-read. I believe they must by law be within +/-10% so to allow for manufacturing tolerances, tyre wear, etc. they will never be spot on. Also, the manufacturer is always likely to err on the side of over reading as a) it keeps you on the "safe-side" of the law, i.e. you're actually going slower than you think you are! and b).....it makes their car look faster than it is!!!!!

This stacks up, I've checked against the speedo in my Vauxhall (UK car) & it's approx 10% higher than the Go. When I check against my Nissan (import) it's a fairly consistant 10mph above the Go between 40mph & 80mph (on the speedo)

Where does the figure of +/-0.1mph come from?!?!?!?!?!?!

Cheers
Nez

Privateer · Posted: Tue Jul 12, 2005 9:16 am Post subject:

Be careful about specifying ± for vehicle speedometer accuracy for UK based vehicles. The speedometer must never under read the speed and can over read the speed by as much as 10%.

On a horizontal road (i.e. no slope), travelling in a straight line, and at a constant speed, your GPS speed will be far more accurate that your vehicle’s speedometer.

However please remember that the SatNav may take as long as one second to report your speed, so as soon as you change either your speed or direction you will have a delay until your speed and direction (i.e. velocity) is constant.

If you are travelling up or down a slope then you are travelling in 3D as opposed to 2D and thus that will add an error to the calculation.

Regards,
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Skippy · Posted: Tue Jul 12, 2005 10:02 am Post subject:

DRW · Posted: Tue Jul 12, 2005 10:07 am Post subject:

Very accurate alongside police laser and radar equipment, I can assure you Wink

HOG · Regular Visitor Joined: Apr 15, 2005 Posts: 114

So as far as speed cameras and radar traps are concerned what happens if my speedo say I am doing 60 and they zap me staing I was doing 62?

DRW · Posted: Tue Jul 12, 2005 10:19 am Post subject:

Nezza42 · Occasional Visitor Joined: Jul 09, 2005 Posts: 15

As you say, a speedo should never underread as supplied by the manufacturer, but if the wrong profile tyre is used, then theoretically it can happen or in the case of a motorbike, a change to the gearing on the chain sprockets may change the speedo reading too.

Using the GPS on a level road at a constant speed seems to me to be an excelant way to verify and/or calibrate your speedo.

Cheers for the replies

Nez

Skippy · Posted: Tue Jul 12, 2005 12:41 pm Post subject:

HOG · Regular Visitor Joined: Apr 15, 2005 Posts: 114

So I have just bought a new Jeep so I can safely presume the speedo is accurate?

Where does one go to check if it is calibrated correctly then?

Privateer · Posted: Tue Jul 12, 2005 2:11 pm Post subject:

Old_Phil · Posted: Sun Jul 17, 2005 9:58 am Post subject:

No HOG, you can safely assume tat your new Jeep speedo is inaccurate!.

Privateer has given the best and most accurate explanation of the speedo debate.

In fact, if you drive so that you travel at the speed limit according to your vehicle speedometer you will never get a speeding ticket. I only wish I had listened to my own advice when I was younger!!!!!
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