I've often wondered about the accuracy of speedos in cars. People throw around a +-10% figure all the time although I've no idea whether really that figure is correct.
Anyway, I've got a 1998 S reg Pug 306, this appears to have a speedo (and odometer/revs) display which is controlled by the ECU (as demonstrated when I had an earth fault and it showed me doing 0-140mph in a few seconds when I was actually just crawling). As a result, would ECU controlled speedos be more or less accurate than a cable controlled one?
I've recently got a TomTom Navigator setup for my PDA including the TomTom Bluetooth GPS mkII unit. This all works well and can display my current speed.
At around 50+mph I've noticed that my speedo appears to consistently under read by about 5mph. Speedo says 50mph, TomTom says 45. Speedo says 70, TomTom says 65. As a result, I've now got in the habit of trusting TomTom and using that to judge my speed, consequently I'm going a little faster than I might have done in the past (speedo reading 55mph when going through 50mph roadworks, but TomTom saying 50).
Are GPS units that accurate? I know they're not terribly precise in a spot reading (few metres) but I would guess that with movement, especially at higher speeds, their 1+m accuracy becomes less important as it's the change in reading which is being used.
Am I being foolish to use my GPS to judge my speed, especially with regard to speed traps on motorways, especially if I might push it towards the 10% + 3mph principle?
Does a constant 5mph over reading by a speedo sound right? I don't notice it so much at lower speeds, although there is still a slight over reading.
One final thing, if my odometer say I've travelled 10 miles, and TomTom says I've travelled 10 miles then the car must be able to accurately measure the distance travelled. Assuming a reliable timer/clock in the ECU, why would the speedo under read, unless it was programmed to do that by the manufacturer? I've often wondered whether speedos are actually designed to under read, especially with ECU controlled speedos.
Any thoughts?
Thanks
D