What speed am I doing

Stephen_Leak

James_N wrote: »

The other thread on here re Speed Reminder Displays made me wonder about the wider picture.

My speedo says (say) 30 dead.
The roadside reminder 29.
My car sat nav 28
My iphone sat nav 27
(Made up example but typical - and differences widen as speed increases)

- Which to believe?

A man with two clocks never knows the right time.

Work on the assumption it's 30.

George_Michael

Notmyrealname wrote: »

Wrong. Construction and Use regulations allow a car speedo to OVERREAD by no more than 10% and a HGV/Coach to overread by no more than 2.5%. They cannot underread at all.

This is one of those urban myths that people still keep saying is fact, yet those same people still keep failing to post anything other than their opinion to back it up.

It has been going on for many years and here is one forum from over 6 yesrs ago with the same 10% maximum over-read error being questioned, and the correct figures being linked to in order to dispel the myth.
http://forums.pepipoo.com/index.php?showtopic=9405

Lum

Hurri wrote: »

As I stated above, this is *not* how typical SatNavs calculate speed.

"GPS receivers typically use carrier phase rate (doppler effect) to calculate speed, and is not just a simple distance/time calculation."

Well the ones I use, and have used in the past tend to lag and/or have the reading jump around if I'm accelerating, braking, cornering or driving through somewhere like Glasgow city centre with it's narrow streets and tall buildings.

The only way I get a consistent speed reading, which is to say one that remains directly proportional to the speedo reading, is when driven in the manner I described in the post that you quoted. I didn't make any suggestion as to how they calculate their speed.

This has been true for the following GPS devices I've owned over the last 9 years: Garmin StreetPilot III, Origin B2, Nokia N97, Nokia N900, HTC Desire Z and Motorola Droid 4.

I'm sure if you got a US Military one it would be a lot more accurate on bends and hills and would probably use dead reckoning to remain accurate even when it lost view of the sky, but in my experience this simply does not happen with consumer devices.

In any case if you're going to do the speedo calibration procedure I suggested you want to be doing it on a long flat straight empty motorway anyway, purely for safety reasons!

LincolnshireYokel

spadoosh wrote: »

Surely the only one your supposed to?!

Your speedo is calibrated to the car (though not 100% accurate) i would imagine adhering to the speed by this method will be most likely to quash a conviction.

Sat navs track over wider distances and can constantly be a few metres out therefore totally unreliable.

Roadside reminders are theyre to give and indication of your speed if your not bright enough to use your speedometer.

Would always go off speedo and should it be out, this will be the only form of suitable evidence i think.

Commecial GPS has 10 metres resolution. The update rate of the satnav also induces a lag in the speed calculation at any instance, but almost any satnav will be massively more accurate than a mechanical speedometer.

Strider590

James_N wrote: »

The other thread on here re Speed Reminder Displays made me wonder about the wider picture.

My speedo says (say) 30 dead.
The roadside reminder 29.
My car sat nav 28
My iphone sat nav 27
(Made up example but typical - and differences widen as speed increases)

- Which to believe?

Are your tyres the correct size?

James_N

Strider590 wrote: »

Are your tyres the correct size?

The question was to create discussion, thanks. There's nothing wrong with my tyres!

smudger1964

fivetide wrote: »

Has it got aftermarket or altered wheels on it? You could have thrown the rolling radius right out.

No its got the standard wheels on think the problem is when you turn the ignition on the speedo needle goes right to the bottom and you have to get up to 10 mph before it gets back up to 0 hence the 10 mph difference..
Its not as bad as it sounds as you get used to doing an indicated20 in a 30 zone etc

And thanks for the other advice if im ever stopped by plod I wont mention this little quirk but as my car is some 14 years old now its just not worth the money sorting it out

Dave_C_2

James_N wrote: »

The other thread on here re Speed Reminder Displays made me wonder about the wider picture.

My speedo says (say) 30 dead.
The roadside reminder 29.
My car sat nav 28
My iphone sat nav 27
(Made up example but typical - and differences widen as speed increases)

- Which to believe?

If you are driving along and comparing speeds between speedo and other devices including an iphone then a speeding conviction is the least of your worries.

Dave

Lum

You can get windscreen mounts for smartphones you know? Makes them work rather well as a satnav.

Robisere

LincolnshireYokel wrote: »

Commecial GPS has 10 metres resolution. The update rate of the satnav also induces a lag in the speed calculation at any instance, but almost any satnav will be massively more accurate than a mechanical speedometer.

Quite right but I hate to tell you as a fellow "Yellowbelly", that today modern cars have had electronic odometers (correct title) for many years. They work by a revolving object (usually but not always a vehicle wheel) passing a sensor. The resulting information is transmitted electronically to the instrument panel. Still not completely accurate, but better than the old cable system. Have a look at the odometer of an old car sometime. The figures will all be slightly out of line. If they are wildly out, someone has altered the figures with an electric drill on the cable or odometer end, by reversing its direction of travel. It always resulted in misaligned digits, similar to mine when I am typing this.