Speed, which is more accurate?

deeranwith · Lifetime Member Joined: Mar 07, 2006 Posts: 32

For a time now I have noticed that both my Sat Navs speed readings are slower than the speedometers in the cars. The Sat Navs both agree with each other and read approx 4-5 mph slower at say 80 mph than the speedometers (one is a digital readout model on a Toyota Yaris).

Which is the more accurate? Is there a significant lapse or error built into the Satellite system? Or is it the poor accuracy of production speedos?

pcaouolte · Posted: Thu Jan 06, 2011 2:57 pm Post subject:

This has been covered a few times (at great length).

The speed displayed on the satnav is more accurate than the speed displayed on the speedometer if they differ. In the UK a speedometer can be callibrated by the manufacturer to show the speed within 0% to +10% therefore if you are travelling at 70mph your satnav will show 70mph but your car speedometer can show anything between 70mph and 77mph.

You shouldn't be travelling at 80mph. Wink

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Paul

deeranwith · Lifetime Member Joined: Mar 07, 2006 Posts: 32

I know, it was for test purposes.

Thanks. Wink

Meter_Man · Lifetime Member Joined: Mar 10, 2008 Posts: 43

MaFt · Posted: Fri Jan 07, 2011 12:12 pm Post subject:

Maybe he was on a test track?

deeranwith · Lifetime Member Joined: Mar 07, 2006 Posts: 32

Good point. Laughing

pcaouolte · Posted: Fri Jan 07, 2011 7:04 pm Post subject:

Re-reading th OP he wasnt doing 80 anyway he was actually doing 4-5mph less than that as indicated by the satnav. Very Happy

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Paul

Skippy

Andy_P · Posted: Sat Jan 08, 2011 4:54 am Post subject:

If you do the maths even being on a fairly steep incline or going round bends will make very little difference to the SatNav's calculated speed.

However the calculations are only done once a second IIRC, so maintaining a constant speed is important (or to put it another way, assume that the speed it displays will lag behind while you are changing speed)

pcaouolte · Posted: Sat Jan 08, 2011 10:16 am Post subject:

cwog · Posted: Sat Jan 08, 2011 8:05 pm Post subject:

I have noticed the indicated speed difference when the tyres on the car were close to the legal limit, after fitting new tyres the Satnav indicated speed and the car speedo indicated speed were very close.

pcaouolte · Posted: Sun Jan 09, 2011 9:42 am Post subject:

Tyre wear must make a difference to indicated mph if the measurement is taken from the number of revolutions that a wheel makes to cover one mile.

On my car a new tyre is 631.9mm diameter and makes 2546.8 revolutions to cover one mile. A tyre worn to the legal limit would be 619.1mm diameter and would need to make 2599.5 revolutions to cover one mile. A difference of 2% between the number of revolutions. This should, I think, give a difference of 2% between the indicated mph on my vehicle with new tyres and with worn tyres.
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Paul

M8TJT · Posted: Sun Jan 09, 2011 10:02 am Post subject:

cwog · Posted: Sun Jan 09, 2011 11:33 am Post subject:

and also the difference in tyre pressure my change effective rolling radius all go to indicate that that the car speedo speed is dependent on many variables hence the latitude in the claimed accuracy of the speedo.

pcaouolte · Posted: Sun Jan 09, 2011 8:06 pm Post subject:

No! I can't be bothered with all that maths Sad

Let's assume that the effects of the rolling radius and centrifugal force and tyre pressures and the amount of clobber in the boot and the direction that the steering wheel is being turned and the tyre temperature and the number of passengers and the road surface and the weather and the number of potholes and anything else that might make a difference are the same in both cases (please) Very Happy

It still seems reasonable to me that worn tyres will give different mph readings to new tyres.

What happens when some of the tyres on the vehicle are worn and others are new I've no idea. Rolling Eyes

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Paul

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