speeding - GPS Detector tells me off.

[tim_n]

Today I purchased a GPS Speed trap detector. It cost £79.99 and can be updated by an optional USB cable and by signing up for a subscription. It already comes loaded with data and if you need to update you can either purchase a kit which allows you to update yourself whenever you want (£30 per year) or you can pay £10 whenever you want and they send you a Jiffy bag to return the device to them for a manual update! This is quite handy because we're already brimming with speed camera devices and updates don't add a huge amount of cameras to the database. I can update once every six months and pay less than most other subscriptions. If I go too fast near a camera or school, it tells me off!

It also gives me a digital readout from the GPS on my current speed. The bizarre thing is that it is 'out' by 2-4mph depending on my speed compared to my analog ford speedo.

So what is wrong? My analog constantly telling me I am doing 30mph or the GPS saying I'm doing 28mph? It's not a faulty unit - my hand held GPS gives exactly the same speed readout! Have the manufacturers decided to lie to us to slow us down?

[XA04]

I may be wrong but I don't think GPS could be 100% accurate to the exact speed anyhow.

[Mike Fishcake]

So what is wrong? My analog constantly telling me I am doing 30mph or the GPS saying I'm doing 28mph? It's not a faulty unit - my hand held GPS gives exactly the same speed readout! Have the manufacturers decided to lie to us to slow us down?

My TomTom speed readout reads about 3-4MPH slower than my car's speedo. I have heard several people mention that car manufacturers do try to slow us down, but I have no idea how true that is.

[Taz]

Your car's speedo always gives you the wrong speed (it always shows you going faster than you really are). Every single car does this as far as I know and it is by design. The most accurate speedo I had was in the Boxster and that was still showing 70MPH when I was actually doing 67MPH.

My current 5-Series speedo shows me doing 70MPH when i'm actually doing 66MPH.

The GPS will give you the true speed.

[Howard]

Yep speedos are always fast. The GPS is always accurate. Of course it'll be more accurate if you've been travelling at that speed for a while, it'll get erratic in stop-start traffic for example. Mine reads about 70MPh when I'm doing about 65 in reality.

And where the hell did you get a GPS detector for £80?

[yamangman]

GPS arn't that accurate. You'd need some sort of intertial navigation to get a true speed.

[tim_n]

Woolworths. Two different models are available, the Inforad and the Talex. The inforad has three LEDs and just beeps if you exceed the speed limit but has free updates for a year and comes with all the kit to update it.

The Talex has a LCD screen that gives you a readout of the time when stationary or finding satelites and when in motion your current speed. When you approach something it gives you a description of what you are approaching (gatso/fixed/specs/mobile etc) and what the current limit is. If you're speeding it tells you to slow down. If you're in a blackspot, it also tells you to slow down if you're speeding. When you leave the danger area it beeps to tell you that you're clear. Unfortunately the 'lite' version doesn't have the cable or the subscription, but it does come preloaded with the data which currently seems pretty accurate. That's the one I went for. You can pay £150 for the same thing with the cable, but since the cable is £34.99 with a years updates, it makes no sense. They also do a mail in option for £10 a time - you send it in a jiffy bag they supply then they return it to you updated!

I couldn't believe some of the places it pointed out on the way home today - what got me was it doesnt tell you when you're going past a fixed camera in the wrong direction - therefore less false results

[tim_n]

I presume that GPS gives truer results than a car speedo, but I wonder what base the police measure on? :- ) how are they calibrated?

'sorry officer, my GPS said 30mph but my speedo said 34!

[Howard]

Hmm.. Tempting. Powered from cigarette lighter?


Hmm just remembered why I didn't buy one in the first place.. My polarised windscreen plays havoc with GPS signals

[5lab]

your speedo cant be very accurate because your tyres change in size, dependant on tread and temperatures - therefore one rotation of the wheel (which your speedo works from) varies in lenght. your speedo will be calibrated to the worst-case senario, so you never inadvertantly speed (you wouldnt want to be done for speeding when your car tells you you're going 30).

gps is highly accurate - to either the nearest meter or the nearest 10m. if it checks your progress every 10meters (and you're doing 30m/second) it is able to get a highly accurate readout of speed.

[Mike Fishcake]

My polarised windscreen plays havoc with GPS signals

A mate of mine had that problem - until I turned up with my PDA and bluetooth GPS unit and put it on his back shelf

[Howard]

A mate of mine had that problem - until I turned up with my PDA and bluetooth GPS unit and put it on his back shelf

Yeah I've got an External Antenna for mine.

GPS arn't that accurate. You'd need some sort of intertial navigation to get a true speed.

They are actually

[Konan555]

Most car speedos will have a tendancy to over read.

A good GPS will be pretty accurate at higher speeds, especially if you data log it, hold a constant speed (use the Rev counter) and then check the data out afterwards in a spreadsheet.

Using my Etrx summit locked to about 15ft accuracy It's within 1mph of a Radar Gun anyway.

My results?

One of my speedos is actuall bang on in the legal range (was expecting over read like most cars)

I think the cables out the other one 55mph equates to about 85.

[Slamdog]

i have 3 gps units. 1 on my lappy, one on the pda and a garmin etrex. they all read the same which is 4mph slower than my toyota avensis speedo.

the police use a separate calibrated speedo in the car, very similar to a lorry tachograph so the vehicle reads true speed, although this requires regular recalibration dependant on tyre wear and temperature as has already been pointed out. gps is more accurate unless the us government reduce the resolution which they can do in time of war.

[BlueMagician]

My polarised windscreen plays havoc with GPS signals

I was always warned against using portable GPS (ie without an external antenna) in cars with heating elements in the front screen - not the fact that the screen is polarised.

Also, I've always been told that because speedos work using final drive of a gearbox, speed readout on the clock can vary by as much as 10% because of tire wear/rolling radius variances etc?

I would trust an averaged GPS reading over a consumer speedo setup in a road car anyday.

S.

[[GSV]Trig]

Closest clocks I ever saw were on my cav gsi, it was on a rolling road doing a smidge over 120, when we got the rr readouts back it said 122mph.
Anything over 40mph give me apx 5mph difference on the road in the cally all over the speed range from 50-a bit faster...