Order of the Bath

The speedometers in cars are notoriously inaccurate. This is something we have always known but with the ubiquity of GPS devices with rather more accurate (if lumpy) speed measuring this has become much more obvious. They are always calibrated to read high so that the inaccuracy will never cause you to inadvertently exceed the speed limit.

This is annoying. Like most drivers with some experience I don’t read the speedometer very often but judge it mentally, only checking the dial when changing speeds (or spotting a camera, but that is another matter). However, even then, we are judging our speed by a previously learned experience based on an inaccurate instrument. It is really noticeable when stuck behind someone doing exactly what they think is the speed limit when you know that they could be going 5 or more mph faster.

The law says that speedometers should be between -0% to +10%(+4 km/h) accurate. As they cannot achieve the -0% by design, they always over read a bit. They work by counting revolutions of the wheels and I thought that part of the inaccuracy was due to tyre wear so I did some calculations. I have 195/65R15 tyres; these are nominally 25″ diameter, i.e. about 807 revolutions per mile. If we assume that a new tyre has about 3/8″ of tread and when worn it is reduced to 1/8″ that is 1/2″ loss on the diameter and now 823 revolutions per mile. This is enough to cause the meter to read 2% fast; certainly significant but not as much as the discrepancy that I observe (nearer 6%). Changing your tyre or wheel size or the differential can make even bigger changes if you don’t get the speedometer re-calibrated.

It ought to be possible to make speedometers more accurate. They could use GPS technology—by setting my handheld device to sample quite frequently it becomes relatively smooth and if this were further smoothed by software and translated to an analogue device it could make a good and accurate meter. Alternatively it should be possible to adapt optical mouse technology. These work on any suitable rough surface, the road would be ideal, and the only thing that would need to be changed would be the range up to about 400 mm to the underside of the car rather than 4mm to the mouse mat and this would probably need a more powerful laser. Pointing the device at the tyre would reduce this necessary range. (I thought of this independently but I see that halfbakery has a similar idea—which proves how daft it is)

P.S. The ACPO formula for determining speed violations is currently 10% +2mph. Police car speedometers are calibrated -5% to +0% (the other way around to ordinary ones) so that they don’t think you are going faster than you really are. Camera speed traps are triggered at an even higher point than this but not enough to compensate for thinking you were in a different speed zone. Those annoying electronic flashing “SLOW DOWN” notices seem to be randomly set and very inaccurate, often reporting about the car three or more in front of you. They have no weight in law.

A House of Lords reply indicated that speedometers are actually manufactured to a -0% to +5%(+10 km/h) tolerance which over-reads still more, not meeting the legal requirement until 120 km/h (about 70mph). In theory when you think you are doing 30 you could be only doing 24 mph though I don’t think many are that bad.

There is virtually no information about this device anywhere. I’m sure that there was a promise of a support web site when I bought it, but there is no sign of anything. The Dixons group (Dixons, PC World and Currys) are the only retailers in the UK and they know nothing about them nor do their parts/accessories agency, Partmaster. There is not much about this device on the various GPS forums yet either.

The Advent GPS 400 is made by Medion as the, now discontinued, PNA 400 re-badged. Their web shop has some accessories including the external antenna and a Traffic Message receiver which I presume work OK with this model.

The box says it uses Navteq software but in fact the software is Navigon 5 (I think v5.1) but with a few modifications, mostly cosmetic; the maps are by Navteq. The only features that I can see are missing are the on-screen qwerty keyboard, signpost information, the info bar and speed limit display and some that would only apply to a PDA device. There is an upgrade to v5.2 on the Navigon download site but I don’t know if it would work with this device. Similarly they have maps available (at a price), including North America, and I imagine these would work with the device. Navteq only sell maps direct for manufacturer’s in-car systems.

Performance

This is not a comparison with other devices, only having used a hand held GPSr before. Some of the faults may be unique but others may be common to all such systems.

• The physical device seems to have been designed for a left-hander. The battery bulge/hand grip is on the right and the stylus withdraws from the back left. If you use it the natural way around your index finger tends to switch the screen off or eject the memory card.
• There is very little in the documentation about the external buttons. Some of the functions are obvious but others are not. The sockets for headphones and external power are close together and very similar, fortunately the plugs don’t engage in the wrong socket. A full set of connecting cables, a windscreen mount and a carry case are provided.
• Getting started proved very easy following the large format idiot’s guide enclosed.
• Entering destinations is pretty straight forward so long as you know the full address. It only accepts partial post codes and if this covers more than one village or district then you need to select one before going on to enter the street. In some cases the best bet is to get close and then use the map.
• When navigating, the directions given are clear and there is plenty of volume from a voice we have dubbed “Sat. Nav. Lady.” Perhaps she says “Please” a little too often but otherwise there are few problems. There is no choice of voice but the volume is adjustable. The use of the phrase “Bear left/right” is sometimes ambiguous meaning either to turn off or just a bend in the road. I think that this is because the system has no concept of priority at road junctions. The A4137 junction with the A48 on route from Hereford to Monmouth was not announced at all.
• It is very good and quick at recalculating routes if you make a mistake, or deliberately overrule the instructions, and she doesn’t complain either <g>
• There is a tendency on twisty roads for the apparent position to wander off the road. This can lead to delayed or completely incorrect instructions; in the worst case to “Make a U turn when possible.” I am hoping that this is down to the poor reception behind our metallised windscreen and an external aerial will solve it.
• The currency of the (UK) map varies. Some very new features are there but some older ones are not. A few junctions are missing altogether (e.g. Sussex Place junction in St. Paul’s, Bristol) and it doesn’t acknowledge mini roundabouts at all. The weakest aspect of the maps is the POI (Points of Interest) data. Many of the petrol stations and garages are incorrect; either missing (from the map), wrong or closed.
• There doesn’t seem to be any recognition of the quality of roads beyond their classification and sometimes speed limit. Examples are: the A5 between Llangollen and Bangor is very twisty, narrow and slow despite its trunk designation. It is preferred over the A55 Expressway on a journey from Shrewsbury to Holyhead. It also likes the A466 Wye valley road. All routes from our house in Redland to the M32 seem to use Brookfield Lane, a very minor residential road which I wouldn’t dream of taking a car down unless I had to, and then turn right onto the busy A38!
• Possibly for the same reason, the ETAs given are rather optimistic. It expects you to be able to achieve the designated speed limit at all times and no allowance is made for negotiating junctions. Some adjustment seems to be made for built up areas and the target speed reduced to 20 or 25mph but even that is not enough with modern traffic. I would allow at least 10% extra time for any journey and more if the traffic was likely to be busy.
• It thinks that the quickest route from Bristol to Bromley is via Central London, ignoring both the M25 and the South Circular. As this was our first use of the device ,we chickened out and don’t know exactly which route it was planning to use. I thought that we had said avoid tolls and expected it to not go into the Congestion Zone but we may have got it wrong. Similarly it has no qualms about taking us over the Severn Bridges in either direction.
• It comes with the UK maps preloaded and street level maps for the rest of Europe on CD. Together with the large 512MB removable memory card this gives it more potential than many devices on the market. It also runs one of the PDA versions of Windows and a copy of ActiveSync is provided giving the potential to run other software. It comes with an MP3 player which we are not particularly interested in but it may have an afterlife as a picture viewer when the navigation features cease to be useful.

Conclusion

Despite some of the rather negative comments we rather like the device. It is easy to use and very good for travelling to places which you don’t know, just that you can usually do better if you do know the roads, however the difference is rarely more than a few minutes.