how accurate are speed monitors

suffolkb

On Sunday,I was driving towards Doncaster racecourse.It was a 40 limit and according to my speedo,I was doing 40.A sign at the side of the road showed my speed as 35 - a big difference.Is the speedo on my Fiat seicento that inaccurate,or the sign (or both)?

Bokken_2

yes the speedometer is normally upto 5% out.This is factored into the calabrations on the speed cameras and allows you to exceed the stated speed by 10% ie.33mph in a 30,44mph in a 40 etc.Local forces also give you a few extra just to make sure it is worth their while prosecuting you.
So 30 limit would give you 30mph+ 10%= 33mph + another 3/5 so between 36-38mph.
Any way it sound like your sign was a safety speed advisorary sign not a speed camera.
Some times the speed camera i s further down the road,just slow down is best.

suffolkb

Yes,it was just a sign to tell you your speed and not one of the speed cameras.The one I`ve seen around here in Suffolk shows your speed and either a smiley face or a frown depending on whether you are under or over the limit.With that one,my speedo agreed with the sign.

balsingh

I know for a fact that some speedos are very innacurate. Generally, the faster you go, the more inaacuate the speedo gets. I have been told about Nissan Micras which are showing 40 when they are only doing 30 .... thats over 30% out.

scheming_gypsy

Never realised the speed monitors showed you your speed. They've always said slow down when i've approached them

m00nie

scheming_gypsy wrote:

Never realised the speed monitors showed you your speed. They've always said slow down when i've approached them

you have probally never been going slow enough to read it...

Woby_Tide

using a GPS SatNav in car I reckon at 70mph my speedo is showing 2-3mph out(faster)

Wig

balsingh wrote:

I know for a fact that some speedos are very innacurate. Generally, the faster you go, the more inaacuate the speedo gets. I have been told about Nissan Micras which are showing 40 when they are only doing 30 .... thats over 30% out.

The way I have always understood it, is that an analogue speedo is calibrated to one speed (usually 60mph) below that speed and you are travelling faster than indicated above that speed and you are travelling slower than indicated.

Digital speedos are unaffected by this.

zappahey

Wig wrote:

The way I have always understood it, is that an analogue speedo is calibrated to one speed (usually 60mph) below that speed and you are travelling faster than indicated above that speed and you are travelling slower than indicated.

Digital speedos are unaffected by this.

Complete nonsense. According to the law, a speedo may not under read but can over read by 10%

the_devil_made_me_do_it

A friend of mine was caught "speeding" a couple of years ago. He asked the policeman to show him the calibaration certificate for the speed camera. The policeman got a bit of a shock and sent my friend on his way.

Basically, everytime the police set up a speed camera, they are supposed to calibarate it according to the area, positioning ect.. obviously this policeman hadn't done that, so he couldn't argue the case with my friend. :T

VincentVega

I've found them to be fairly accurate, but it really depends on your car and how its speedometer is calibrated. Near where I work (Wolverhampton) they put up a thing that flashes the speed limit (30) at you if you exceed it. Going at 31mph doesn't set it off, but go much higher (not that I do!) and it will go off.

You don't have to worry about being sent a fine, unless there's a policeman 50 metres down the road. IMHO, these things are a great idea. You don't get pulled over for exceeding the limit by 3mph or whatever and I find it fun (sad, I know) to try never to get flashed at.