charlie_morris_minor wrote:i can not find the post, but i am sure someone on here has stripped the paint off the face of their speedo and went for a drive without the glass in and got a friend to mark the 20 , 30 marks etc might be an option
Charlie - my speedo needle sometimes sways up to 5mph either way - which would be the exact speed 'point'
Sat nav's are very inacurate as the us military found out when developing the GPS system they didn't include a clock in the programing so all the GPS satalites they put were inacurate a plotting a location by 380,000ft. Thays why a sat nav uses a minimum of 3 satalites the speed shown on the sat nav is an average time of each satalite plot and the relitive distance traveled over that time so you can be showing the wrong the speed quite easily on a sat nav.
Neil G in answer to your quote on vehicle consruction and use regs. All speedos must be accurate to witin +/- 1% at 30 MPH and yes it must never under read then 10% there after. This is because the speed limit on urban roads is 30 MPH when the regs were introduce and then ammened i think early 1950's. They only had 2 speed limits in force 30 and the national speed limit of 60, which is why you see two bolder lines on some older speedos at 30 and 60. Tt was considered at the time that cars would rarely go out into the country (weekends) and would be more lickly to be in built up areas so the the +/- 1% for 30 was introduced.
Obviously as has been said on here the older the car the less reliable the speedo is due to wear. My speedo on my series II doesn't show a speed until it's around 30 anyway between 0 & 30 it's just bouncing up and down. i've have the speed checked by a friendly traffic officer using VASCAR and at 40 over a messured mile it recorded 42.3 MPH.
I wouldn't worry to much when was the last time you saw a traffic police officer on the road ?
Alan Pearse
Register of the
Series II
Saloon & Convertible
Register
so, as we've decided the normal speedo isn't accurate, you can't check it with a friends car at the same speed as theirs wont be accurate either. you can't use a satnav as that's not accurate either. the only way to check your speedo accuracy accurately is with a rolling road, a friendly traffic officer, or by getting a speeding ticket.
also, those clever signs that tell you how fast you're going. the last time i went past one, my speedo was reading 38... the sign said 7... not 37, just 7!
Speedo's in modern cars and sat navs are perfectly acceptable and accurate enough for anyone who uses a car normally. Calibrating your old cars speedo against a decent sat nav is also perfectly adequate and the easiest way to do it for most people..... Simples....
[sig]8426[/sig]
Compare the Minors - Simples !! http://mog.myfreeforum.org/index.php
Speedos have NEVER been accurate to +/- 1% at 30 mph!!!! And within the limits we are interested in (+/- 0.1 mph ) - GPS is deadly accurate for speed. And I find the flashing signs are also very accurate - given that the car may not be holding a steady speed anyway. Most modern cars appear to read 3 mph too high - all the way up the speed range. In other words - it's a fixed offset, not a progressive error.
"2001 No. 25
ROAD TRAFFIC
The Motor Vehicles (Approval) Regulations 2001
SCHEDULE 3
APPROVAL REQUIREMENTS FOR RELEVANT VEHICLES
Section 19.3
For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph."
Alan P, the satellites may or may not include clocks, but the sat nav does and that is extremely accurate. Position is accurate to 7m. Given that the sat nav speed doesn't change up and down when driving at a steady speed then it is damped to negate that error. (The variation can never be cumulative) So, unless there is a fundamental problem with the device, we can be reasonably confident that it will be accurate to within +/-1 mph (and probably +/-0.1mph). That is on flat ground, in a straight line and at constant speed!
Maybe that is not accurate enough for the US military, but it's good enough for me!
1956 Morris Minor Series II
1959 MGA 1600 Roadster
1966 Jaguar Mk2 3.8 MOD
