Satellite Lag, you need to be at a constant speed for a few seconds.

Olly SD4 HSE wrote:

I doubt if either are more than 98% accurate.

Speedo tend to over do this for the reasons stated.

When you think of the distance you are from the satelite on your GPS don't you think there is some rounding up or down in the mathematics it is doing and allowing for the time the signal takes to and from your vehicle.

The car only receives GPS signals, it doesn't transmit any signal to the satellites. Also GPS speed readout is only accurate if the car is travelling on level ground so not so good going up or down hills. The average car speedo is more than accurate enough for the job it has to do. Mine is within 2 or 3 MPH of a TOM TOM readout at 70 MPH and always reads greater than the GPS.

Kickstart wrote:

Olly SD4 HSE wrote:

I doubt if either are more than 98% accurate.

Speedo tend to over do this for the reasons stated.

When you think of the distance you are from the satelite on your GPS don't you think there is some rounding up or down in the mathematics it is doing and allowing for the time the signal takes to and from your vehicle.

Radio waves travel at the speed of light.

If you wish, I can easily discuss Doppler effects. I am a licenced amateur radio astronomer with a particular interest in EME. That's a bit embarrassing isn't it?

Hold on, hold on chaps......

I need to get my popcorn, and get comfy.. Standby....










Right.....as you were. Carry on.

It's fine. I thought it was funny.

What's the chances of fronting up about Doppler effects with an unknown chap on a land rover forum who turns out to be a radio astronomer?

Last edited by Kickstart on 3rd Mar 2014 7:26 pm. Edited 1 time in total

I'd love to read a discussion on the 'doppler' effect and phaseshift please

Edit: I'm still googling EME

Last edited by EYorkshire on 3rd Mar 2014 7:28 pm. Edited 1 time in total

I've had around 25 cars and the speedo has read over on every one of them. I thought they were designed that way to be on the safe side. It also means your car "wears out" 5-10% quicker and your services come around 5-10% quicker as well so the manufacturer is quite happy about that. If the speedo is wrong then the odometer is wrong also.

Kickstart wrote:

If you wish, I can easily discuss Doppler effects. I am a licenced amateur radio astronomer with a particular interest in EME. That's a bit embarrassing isn't it?

No need for you to feel embarresed, radio communications has been a hobby of mine since 1981 firstly CB, then Ham VHF followed by Ham HF although mainly now SWL due to antenna restrictions in my house deeds because of my home being close to a major RAF base. However a homemade reduced G5RV still gives resonable pleasure.

So without trying to pick an internet fight you as you will know as well as I the margins used in GPS and the quality in cheaper mobile units does not match military grade equipment. So in short I will go back to my original 98% accurate statement and now ammend that to 99% if it makes you happy.

This is my last post on this topic as I can't really be bothered as it is not that important to me

mikehzz wrote:

If the speedo is wrong then the odometer is wrong also.

Not necessarily. If speedo is ment to show a little over by manufacturer, there's really no point to do that with odometer.
I've checked this once on a long trip. My Volvo V40 speedo displayed almost on every speed +4kph. On a long trip from A to B, I checked the distance from Google maps and I also had a GPS with trip distance in my car (Magellan Sport Track) and the odometer discrepancy was only 0,7% (+6km) on over 900kms.

Easy on road test to show if speedo error is done on purpose by manufacturer:
Drive on a fairly flat road with cruise control on. (so that speed stays really steady) Reset average speed at on board computer and wait for a while until it calculates average speed again. So the ave reading should be exactly the same than on speedo? You might be surprised... not!

Anyway, if speedo error reading bothers you, go for a larger diameter tyres.
Just do some comparing with tyre size calculator (like this one) and look what happens to speed.
For example, my last car before Freelander2 was Volvo V70, which has normally 225/50-17 tyres. I bought a new set, they were 215/55-17 and they are slightly higher. With new ones, speedo showed only +1kph.

You get longer service intervals also!
Anyway, sure there is differences between different manufacturers tyre diameters in same tyre size.
So you might be really lucky or not so lucky, depending on your choice.

BTW, my Landy's speedo is almost 100% correct with normal 235/65-17 Nokian spike tyres
On spring I'll see what happens to speedo when summer Conti's go under the wagon...

standard gps signal isnt very accurate . easily accurate enough for navigation etc but. in precision farming etc where you need accuracy down to 1 or 2 cm and repeatabilty constanty the gps signal has to be corrected against a base station (typically mounted on high barn etc etc).
another system triangulates your mobile phone signal and compares that to the gps signal

http://www.kowoma.de/en/gps/waas_egnos.htm

http://en.wikipedia.org/wiki/StarFire_%28navigation_system%29 At work
At home

"I can't always believe facts I read on the web" - Charles Dickens

winner by default of the tractor vs caravan race

Olly SD4 HSE wrote:

Kickstart wrote:

If you wish, I can easily discuss Doppler effects. I am a licenced amateur radio astronomer with a particular interest in EME. That's a bit embarrassing isn't it?

No need for you to feel embarresed, radio communications has been a hobby of mine since 1981 firstly CB, then Ham VHF followed by Ham HF although mainly now SWL due to antenna restrictions in my house deeds because of my home being close to a major RAF base. However a homemade reduced G5RV still gives resonable pleasure.

So without trying to pick an internet fight you as you will know as well as I the margins used in GPS and the quality in cheaper mobile units does not match military grade equipment. So in short I will go back to my original 98% accurate statement and now ammend that to 99% if it makes you happy.

This is my last post on this topic as I can't really be bothered as it is not that important to me

You need to re read my post.

In the spirit of the forum. Google then build a G5IJ, far more use to you as a SWL than a G5RV in a restricted space.

Apologies to non Radio types.

Here's an interesting 'doppler effect' question

Q: What is the simplest way to observe the optical Doppler effect?
A: Go out at and look at cars. The lights of the ones approaching you are white, while the lights of the ones moving away from you are red.