OT: Biodiesel

Isn't the tacho also the speedo? Not that I've looked at many of these things but the ones I have the paper recording disc goes behind the speedo display...

As all tachos have to be, and checked at intervals as well I expect. The average car speedo isn't calibarated.

On a straight road going at a steady speed I think I'd trust the satnav speed indication more than a cars uncalibrated speedo. Indeed I have heard of people using satnav logs to show that they weren't speeding...

Making the assumption that the satnav has taken a straight line between the points used?

If there is a changing speed yes. The calculation will be assuming a steady speed between its reference points.

The GPS sattelites (note: more than one) only send a signal that the reciever uses to work out where it is. It can do this pretty accurately provided the Rx can "see" a goodly number off sattelites. The speed is calculated from where it was to where it is and the time taken to travel that *straight line* distance.

Unless you have a pretty powerful car that can do 60-70 in a second or less then the speed indicated by the GPS on a motorway will track fairly well your speedo indicated spped. A GPS Rx gets positional fixes about every second or so. So the speed calaculation is done every second or so but note that this speed is not the current speed it is the one calculated from the previous two positional fixes.

Try walking and stopping/starting with a GPS and see the delay in positional fixes and the lag in displayed speed.

No, it is a separate instrument with its own speed and time display. The fuel consumption computer that Martin mentioned on his van isn't available when you have a tachograph fitted, it is disabled.

No, but as they are all electronic now, and fed from a digital sender, same one that feeds the tachograph, they should be pretty accurate compared with the old magnetically driven types.

Yes, on a straight road etc etc., but driving in Scotland last week across from Glasgow to Kendoon, the last 40 miles was anything but straight!

I don't think my TomTom Mobile5 has a speed display, or at least I haven't noticed anything like that.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk

I have TomTom 5 on my PDA having a speed display is in the Status Bar Preferences under Change Preferences. I find that my cars speedo is 2 or

3mph above what the GPS says, I'd rather trust the GPS under steady state conditions or even on a "bendy" A road provided one is going at traveling at a resonable speed. Because of the fairly fast (second or so) positional updates bends would have to be pretty major for the actual path and direct path to be significantly different.

No, it's too complicated for me to understand properly but a a chap I know with who's career gave him good reason to understand these things posted on uk.rec.maintenance message id snipped-for-privacy@4ax.com

in which he explains that using the signals from the satellites a velocity is directly derived without reference to a timed change in position between two points.

See above, it doesn't. The discrepancy is due to the display only being updated every second or so, which is why you can come to a halt and still see a speed being shown. For all intents and purposes an accurately calibrated speedometer will give a better indication of speed but a speedometer can still be in spec and under read by 10%.

AJH

I fail to understand how a satellite system which is being updated, and cannot take into account the twists and bends of a road, can be more accurate than devices driven directly by the wheels of the vehicle, with no breaks in transmission etc etc.

All speedos are electronic these days, and although technically we are allowed the 10% error, in practice they are pretty good, albeit allowing for changing tyre diameters will compromise that accuracy.

Linear speed, yes, with reservations on a motorway for example, but otherwise I remain deeply sceptical of the claim for satnav.

Peter

-- Peter A Forbes Prepair Ltd, Rushden, UK snipped-for-privacy@easynet.co.uk

Peter you didn't read my post. You said the satnav depended on two different positions to compute a speed between them, I said it did not and referred you to Peter Parry's post on the subject.

I did not claim the satnav was necessarily providing a more accurate speed indication than a speedometer and I explained why.

AJH

"AJH" wrote

Found the thread at last, perhaps this link will work (or more likely not!):-

PP asserts that the gps system calculates velocity by doppler shift of the carrier - I'm impressed!

Nick H

"Nick H" wrote (snip):-

A little brain food on the matter:-

Nick H.

He's a helpful bloke, I'm an oik with little respect for the establishment but I know two people that the powers that be have chosen to honour and it couldn't have gone to better people.

Are you offering a summary?

AJH

Gentlemen,

Well they livened us all up didn't it, at least I have a better idea how it works, I'm off to London on the train tomorrow I may take the Sat-Nav along for the ride.

Mart>

Where did I say that??

Peter

-- Peter A Forbes Prepair Ltd, Rushden, UK snipped-for-privacy@easynet.co.uk

I may have mis interpreted what you meant by :

"I wouldn't personally take too much notice of satnav speed figures, as they can only go by linear distance between satellite accesses, and don't take into account other factors like bends and roundabouts."

but I took it to mean you thought the speed was computed by the linear distance between two satellite fixes.

No argument though, we're both agreed a good speedo is more usable, I suppose the only real reason for knowing your speed is to comply with speed limits as any decisions as to what speed is otherwise appropriate come from your senses.

AJH

Assuming a TomTom like devive it will be interesting to see if it will track along the railway lines it knows about or insist on trying to be on a road. That's always assuming you can get enough signal through the metalised windows.

On Mon, 21 Jan 2008 18:03:37 +0000 (GMT), "Dave Liquorice" finished tucking into their plate of fish, chips and mushy peas. Wiping their mouths, they swiggged the last of their cup of tea, paid the bill and wrote::

I tried that with the GPS in my phone. Once it had acquired a satellite or two (which takes an age) it tracked my position along the railway track but persisted in telling me "Not on a Road" - hardly surprising!

Brian L Dominic

Web Site:

Newsgroup readers should note that the reply-to address is NOT read: To email me, please send to brian(dot)dominic(at)tiscali(dot)co(dot)uk

What an interesting thread! I learned stuff I didn't know & found a greater respect for SatNavs - already pretty high when you consider what they do & how little they cost.

I wonder if wear and tear makes much difference? My 105,000 mile BMW 530i records 81 mph on the speedo and 78 mph on the SatNav. My previous car (F reg 535 iSE) was exactly the same. Both E34's but about 100,000 miles difference in mileage.

With more points than is comfortable, I stick to an indicated 80mph on motorways these days & even closer to roadworks limits.

Couple of things - can average speed cameras see in the dark and do they REALLY only cover a lane apiece??

Almost certainly they will work well into the infra red so will pick up the number plate. In about 1968 my father was in the home office when they were testing materials for reflective plates, it was well before digital cameras so they were considering film, and one of the reasons for the bright yellow rear plate was for better contrast for the cameras they were intending to use.

From what I've read it was more to do with how they presented the system for some sort of legal compliance. The cameras cover multiple lanes but are only compliant if they catch the same car in the same lane, so shifting lanes between cameras made it impossible to prosecute, I expect they've resubmitted the system for multiple lanes by now.

Andrew Heggie