Which speedometer is most accurate?

tonetheteach · Occasional Visitor Joined: Feb 13, 2006 Posts: 6

Greetings-
I have recently bought a TomTom One and when my wife and I went out for a drive today I noticed that at the bottom of the TomTom screen it showed the speed at which we were travelling. However, when comparing this with the speedometer of our Ford Focus it was noticeable that the car registered a speed of three or 4 mph greater than that shown on the TomTom screen.

Which, if either, is likely to be the most accurate? I hope someone can help with this as it will put a stop to a little bit of disagreement!

Best wishes --tonetheteach.

Oldboy · Posted: Sun Mar 05, 2006 11:26 pm Post subject:

The GPS unit will always display the most accurate speed.

There are several threads on the subject, on the Website. If you care to read them, use the Search icon at the top of the page. Try 'speedo' as input. ;)
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Richard

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999tommo · Posted: Sun Mar 05, 2006 11:26 pm Post subject:

Hi Tonetheteach,

My employment involves driving vehicles fitted with calibrated speedometers and other speed detection equipment. I have used my TTOne at work and can verify that the speed shown on the TT is extremely accurate, providing you have a good satellite fix. Incidentally I can also confirm that a Focus speedo is approximately 3-4 mph over-optimistic. A Mondeo is about 2 mph over. Most cars do read higher than actual speed to give motorists a fighting chance at keeping legal (ish).
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Tommo...

Regularly absent, but still here in spirit !

tonetheteach · Occasional Visitor Joined: Feb 13, 2006 Posts: 6

Thanks for such a prompt reply- I thought GPS would be more accurate than car speedo. One up for the bloke who bought TomTom One in the first place-best wishes- tonetheteach.

Unplugged · Posted: Wed Mar 08, 2006 3:30 am Post subject:

Well if you start at one end of a street it shows that end and then walk to te other end of a street it shows the other end so its logical to assume that the sat nav has the exact distance in between both points so can work out with far grater accuracy something that is just timing the revolutions of the wheels or gear rotations whatever they come off nowadays.

linrew · Posted: Wed Mar 08, 2006 9:45 pm Post subject:

hi

having had my tomtom for nearly 18 months now it has been in many cars/vans

i have found that fords seem to have the least accurate speedo (at least 10 percent out IE speedo says 70 tomtom says 63) and chrysler/dodge seem to be fairly accurate (about 2 miles per hour out)

i read somewhere that tomtom is accurate to 0.1 mph

linrew

Skippy · Posted: Thu Mar 09, 2006 2:57 pm Post subject:

BillyHey · Posted: Wed Mar 15, 2006 12:12 pm Post subject:

If you want a more accurate reading from the Focus, hold down the 'trip' mileage reset button as you turn on the ignition. You get into a nice diagnostic mode and pressing the trip reset button will cycle you through some options, one of which is the actual reading from the gearbox speed sensor. The 'needle' speedo is usually 4-5mph out from this in a focus, provided you haven't Kev'd your car up and changed the overall wheel size

It comes out of diagnostic mode when you turn off the ignition, so don't worry about breaking anything.

All car speedos are set to overread, and never to underread for legal reasons.

tonetheteach · Occasional Visitor Joined: Feb 13, 2006 Posts: 6

Thanks for that BillyHey- You learn something new every day-
Best Wishes - tonetheteach.

Antoeknee · Lifetime Member Joined: Jul 29, 2005 Posts: 267

My Mondeo is approx 10% overreading al through speed range compared to TT.

So at 40mph on TT its 44 on speedo and at 70 its 77ish on the speedo.

I've been passing speed traps using the TT speed and not had any nasty letters yet.

Tumbleweed · Frequent Visitor Joined: Aug 30, 2004 Posts: 315

smstextaddict · Posted: Wed Mar 15, 2006 11:44 pm Post subject:

you can show your diagnostic speed on a vauxhall by holding the buttons in whilst moving for the clock area, you will have to play about with them (which ones to press) as i cant remember off hand but it will tell you in kph your speed.

mrzipy · Regular Visitor Joined: Mar 10, 2006 Posts: 79

I drive a Citroen C4 and the TomTom speed is usually dead on or at most 1mph behind the cars speedo.

The C4 has an all digital speedo.

TomDavison · Posted: Thu Mar 16, 2006 5:59 pm Post subject:

Satnav speed only works correctly if you are travelling in a straight line, and unless you have a specially calibrated GPS, (usually for aircraft use), if you are on a level road.

When you go round a corner, the satnav measures the straight line distance between two points over a set time, and calculates the speed based on that. So it tends to under-read.

Normal satnav only measures in 2 dimensions, so it underreads when you are on a hill, whichever way you are travelling. The steeper the hill, the bigger the error. (bit like looking at an ordnance survey map.) As an extreme, if you drove off a cliff and went straight down, the satnav would say you had zero speed, since you aren't moving over the ground.

What it isn't subject to are errors due to variation in wheel diameter (New tyres v worn tyres), or the mechanical errors in the car speedo system.

TomDavison · Posted: Thu Mar 16, 2006 6:02 pm Post subject:

Forgot to mention that it also only works accurately at a steady speed. If you are accelerating or decelerating, it gives the average speed over the recording period, so always lags behind the true speed.

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