I have got a new Tomtom XL for christmas and got to test it a bit.
On my own car I have discovered if my speedo reads 30mph I am actually doing 27.
My dads is identical but has bigger wheels 14" instead of 13", his reads 28 when he is doing 30.
And the best bit, I tested it on a train, was on a Class 91 with rather fantastic reburbished MK4 Mallard rolling stock, these are simply the most comfortable trains I have ever been on, and I have been on TGVs and the german and dutch stock.
The train reached a 100mph limit betweeen Doncaster and Leeds it felt like I was doing 70mph, so I got my TomTom out and amazingly it recorded a steady speed of 100mph for 5 minutes I had it out.
So my obersations:-
Even cars as old as 1996 seem to have speedos that underead rather than over read (which is different to cars designed say in the 80';s or earlier)
The speedo seems very accurate as longs as you're doing a steady speed, it does not seem to cope well with constant slowing down and speeding up due to the gaps in the tracking.
Just thought I would share my observations as a lot of questions often come up regarding this.
Also on the 100 mile journey by car this morning (I was a passanger) the speed limits the sat nav displayed were correct, but there was one large B road with a 60 limit the sat nav did not know the limit for.
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Rattle - try it on a 737 flying between Heathrow and Glasgow
Specially with the speed cameras loaded.
525mph on a 60 mph limit really makes it squeak with indignation.
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Haha how did it keep up redrawing the maps? Even at 100mph it had to recalculate the roads every few seconds, but I think it got confused as there was no roads a lot of the time. Then i accidently turned the sound on, I got a few funny looks "turn round, turn around then lern left at 300 yards"
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Rattle
Speedos have to not under read so therefore over read and have for ages....
The great thing is is knowing this, drive as if you do not. You'll not be caught for speeding as you're not.
I managed to drive from the GMEX car park to home last night and only had five red lights that I had to stop for. Two were immediately outside the car park... got stuck there before so never again.
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I meant overread sorry to confuse you :) tired.
Indeed I suspect it my speedo must have overead a bit anyway because that time when I had a mad panic when I was doing 35mph outside the Gorse Hill pub on Talbot road right nex to the speed camera (I thought it was a 30mph limit and learn my lesson on that, I always asume its 30 now unless I see a 40 sign).
I will still continue to drive at 30mph in 30 zones, even though I will know I am doing 27, 27 is usualy fast enough and with that policy allowing for margin for error i.e I go slightly over 30 I would probably have to drive though at 38-40mph to get caught, which means sticking to 30 means I will never get caught out by a speed camera :)
I have horrible memories of that GMEX car park, many years ago was on a school tip to GMEX and a coach from that well known company in Fallowfield turned up with this B reg coach this was circa 1997 and it got to the ramp into the car park and the clutch get slipping, was a pretty scary experience as the coach kept rolling back.
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If you drive up any motorway at a steady 70, on your satnav, you will find yourself overtaking vehicles going just a bit slower than you. This is because they are doing 70 on their speedos, which is about 66mph but could actually be as slow as 63. Speedo must be within 10% but must never under-read.
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>This is because they are doing 70 on their speedos
And I presume trucks are limited to 'speedo 60' which equals about 56, or in reality 90kph? I have never found it necessary to do much more than true-60 to pass them unless they are Irish or continental.
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The regulation is -0 +10% on all vehicles in the EU since 1993 ( I think ) In practice it is often in the 8% range with new tyres and less when worn around 6% high. Traffic cop cars have this margin programmed out by the ECU correction maps. My Audi A6 is 7% high on 25% worn fronts. Beware of non standard alloys for the model and the trye width and profile effecting the reading. DO not spend all you time starring at your sat nav. Calibrate your brain not your eyes, that's when the accident happens. Regards Peter
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What it means in terms of speeding is that the claim that you just crept over 30 is more than likely a false one as on most cars would have to be doing an indicated 36/37 before any cameras start flashing. Its a good safety net in my view although at an indicated 80, satnav says my Daihatsu is barely hitting 73.
Funny but I still feel more comfortable going by the speedo than the satnav.
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What I have noticed is how inaccurate the indicated road speed limits can be, and how they contradict the actual signposts that I pass. I'm assuming that these are linked to the map you have but even on my local roads which haven't changed for years the limits are incorrect, often then satnav is warning me that the limit is lower than it really is. It makes me wonder how they would ever bring in policing speeders by a black-box installed in every car, I could see lots of incorrectly issued tickets.
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There is one of those 40 mph speed limit warning flashy things near us at an accident blackspot. If you pass it in one of my cars at a dead on indicated 40 it will spark up petulantly. In my other car it remains unlit at the same indicated speed. Presumably one car has a speedo slightly differently calibrated than the other.
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DO not spend all you time starring at your sat nav. Calibrate your brain not your eyes, that's when the accident happens. Regards Peter
I'd have to disagree with the idea of calibrating your brain - it doesn't work for me.
I get accustomed to travelling at a given speed - say 70 for an hour or two - then coming off the motorway onto roads with 40 and 30 limits it feels like I could get out and walk faster.
The only way of being sure of your speed is to check it periodically. And it's probably safest using the speedo in the car, as it's position is likely to be optimum.
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My dads is identical but has bigger wheels 14" instead of 13", his reads 28 when he is doing 30.
Very easy for him to get nicked then. The perils of putting incorrect wheels and tyres on a vehicle. Hope his insurance company don't find out.
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Martin the wheels were fitted in a lovely factory in Essex back in 1996. They are the completly standard alloys you got on the Ghia X. Mine are the completly standard steel wheels you get on an Encore.
If they were added since the factory there first thing I would have done is removed them. In fact when we bought the car he didn't do know it was a Ghia, and he did nothing but moan the next day about how it makes a huge difference to insurance premiums (which was actually about £100 a year for him).
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I'd have to disagree with the idea of calibrating your brain - it doesn't work for me.
Is out of context, I was referring to not starring at the sat nav rather then 'calibrating your brain' A sat nav is in the near field and as it displays small numerics then you alter your focal length to the detriment of the car in front and other obsticals. You tend to observe the position of the needle on a speedo not the numbers that it is pointing to. Regards Peter
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>>>>I'd have to disagree with the idea of calibrating your brain - it doesn't work for me.
>>Is out of context..
The dyspraxia discussion in IHAQ did rather ring a bell for me.
It's not so much "wrong end of stick" as grasping the stick in the middle, or putting it another way, if there's a wrong way of doing something it's usually my first option.
For me "putting 2 and 2 together" gives twenty two.
However, when I do eventually find the right answer every wrong option will have been exhaustively explored. Quite tiring. Sometimes rewarding.
Apologies to Peter.
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I went through about 4 of Trafford's '30mph 'flash up signs yesterday, Rattle will know them, I was stuck behind 4 or 5 other cars behind a stopping service double decker. We were all travelling at an indicated 25ish mph through them all. All of them flashed at us. This makes me wonder if they are set much lower than the speed limit in a sneaky attempt to slow down the traffic.
Ted
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If the wheels were fitted at the factory then something is wrong somewhere.
It is against the construction and use regs to have an under-reading speedo.
Fitter larger wheels is ok of teh profile of the tyre is correspondingly decreaded to keep the rolling radius the same.
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Martin I think I have confused you. My dads reads 28 when the sat nav says he is doing 30. Mine reads 27.
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Then neither your nor your father's speedos are legal - they are not permitted, by law, to underead SFAIK. See earlier posts re accuracy and the law - Peterd's post at 1048 on 28th
Edited by Armitage Shanks {p} on 28/12/2008 at 15:53
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They may have been "production" parts but if fitted at the assembly plant would also have had the correct speedo drive gears fitted to the gearbox.Now,this is programmed into the ECU.
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Remember the satnav gives your speed across the surface of the planet - not your raod speed. Most of the time no difference, but going up or down hills there will be.
/pedant mode off/:-)
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Sorry!!! I am confusing myself here.
They both overead, if my dad is doing 28mph on the sat nav he is doing 30mph on the speedo! I have confused myself here. Sorry.
My point was they both underead at different levels despite them being the same car, only difference is wheel size.
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>I am confusing myself here ... Indeed :
>They both underread at different levels - No, they don't UNDER-read, they over-read because the car is doing 28 when the speedo reads 30 - that's legal. Got it?
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All forgiven, it was your first post that got me started.
my speedo reads 30mph I am actually doing 27.
his reads 28 when he is doing 30.
Edited by martint123 on 28/12/2008 at 20:41
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Rattle, would be a good idea to 'calibrate' the speedo against the satnav at 30/40/50/60 and 70mph.
You can't assume that because you are going 27 true speed when there's an indicated 30 on the speedometer that there is a built-in 10% difference in the reading.
It may be as small as 5% at 70.
Also, have you checked the odometers for accuracy? That would be quite interesting to find out.
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As someone else said they all show slightly under so if you are really worried about speeding then stick to the speedo speed and you can't go wrong... and if you do creep a couple of mph over it you'lll also be ok... and I'd have thought that worrying about 100% acuracy of the speedo is the least of Rattle's worries, relax a bit, kiddo!
Edited by b308 on 29/12/2008 at 09:25
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On the cars I've checked sat nav speed vs. car speed the difference at 70mph as a %age is greater than at 30mph. At lower speeds the speedometer is more accurate.
The fact that modern cars' analogue dials are controlled by an ECU it is easy for the manufacturers to do this. I remember my Mondeo's digital speed reading (when in diagnostic mode) was more accurate than the analogue gauge.
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Our C4 has a very accurate speedo - digital - at an indicated 69 mph today, Tomtom was telling me 68 and the new exciting Garmin cycle GPS was saying 68.4/68.6
I'd always given myself about 10% error margin, but it looks like at an indicated 77mph I'm probably doing 75-76 mph.
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>the new exciting Garmin cycle GPS was saying 68.4/68.6
wow I bet your little legs were a blur cycling that fast.
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I think I've melted my knees
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Citroenian (sorry for the lack of an umlaut on the 'e'),
Off topic, would this the Edge 705? I have been looking into buying one and would welcome some feedback as to whether you think it is worth the money - the reviews in the journals are on the whole less than informative. What map are you using City or Topo?
BIG
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It's an Edge 205 and I've not had a chance to play with it properly, save for a car journey this morning - the software installed OK and got the stats back to the PC no problems. The elevation aspect is a bit dodgy as it uses GPS not an altimeter but the rest of it is fine. Simple to use and light it's a neat bit of kit.
Only concern may be the rechargeable on unit battery - needs a Mini USB to recharge - would have preferred some batteries that I could swap on a long ride but I've not ran the unit long enough to find out how long the batteries last as yet.
hth
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"The elevation aspect is a bit dodgy as it uses GPS not an altimeter "
The GPS systems provides info for altitude but most (i.e. car) sat navs do no use or display this. I use OS maps on my PDA and phone and altitude is used. Okay not going to be as accurate as an altimeter.
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Re planes breaking speed limits!!:
Is there a maximum height above a public highway at which traffic regulations apply?
For instance, if a light aircraft follows a road at a speed of 140 mph, is that technically a speeding offfence? If the law is that it considers a vehicle has to be in contact with the road surface, would a small hovercraft travelling along a road bound by speed limits? Would it need to pay tax and insurance? Also, I wonder whether, at a flooded road junction, a rowing boat has to stop at a red light!
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I was once standing at the top of the road pass into Glencoe - two fast Jets came over my head (unheard and unseen from behind) at about 30ft and were immediately below me as they dived into the glen. Never stopped there again after that - what a shock.... I`m sure they would have triggered a road speed camera if so placed.
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Would a push bike be detected by speed cameras or would their not be enough weight/mass? I would love to do get flashed with me riding a bush bike.
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One of the early Gatsos in Edinburgh could certainly be made to flash by pushbikes. Seemed to work better if you....sorry ahem....the rider...... was side by side with a mate. Trickiest bit was ( I'm told ) timing the reverse Churchillian salute to coincide with the flash while maintaining speed and control.....
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I would imagine you would have to be quiet fit too, as I kid I had a speedo and once did 34mph on the pavement, I spent the entitre holiday days training for that. Pretty dumb and stupid looking back now.
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Pleased to say I have always been quietly fit......
;-)
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Oilrag - There is a stretch of the M6 near Tebay where the valley falls away to your left if you are southbound. It used to be quite a common and indeed thrilling sight to see RAF jets below the level of the road during their training excercises. Don't seem to have seen that in recent years though. Probably banned or something.
The other place it was not unusual to come into close contact with them was on the A701 Moffat to Penicuik road.
I was brought up pretty much at the end of the runway for RAF Turnhouse. The sight and sound of Lightnings being pointed vertically on afterburner was a real treat to watch as kids. I expect that's banned too now.
Can't think of a connection with the thread topic so I'll shut up now......
;-)
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Also down the Llanberis Pass, Humph.
Ted
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I expect RAF jets have sat navs and maybe even speed limits but I reckon we're still on thin ice Ted ! Pity really.......
I once drove to Llanberis.
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GPS uses a mathematical model of the earth's surface that is not entirely accurate. Thus actual elevations won't be very accurate, but relative ones will be OK. So the GPS might be wrong about your altitude when, for example, on the beach, but it'll be fairly accurate about the height gained in getting to the top of the hill inland.
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