How Accurate Is Your Speedo ?

G4EIY · Unread post by **G4EIY** » 30 Nov 2005, 20:06

Hi Folks,

I have often wondered how accurate the speedo was on my car.

But never got round to doing a check using a measured distance (Mororway Juntions) against time say doing an indicated 60mph.

Then the other day I borowed my sons GPS device, and I was able to do a direct comparison.

Both the speedo's on my Bx's were reading 10% fast at 30MPH, and 7% fast at 70 MPH.

I thought that this was a little strange, expecting them to both read high, as I thought the springs in the speedo heads would have relaxed a little due to the miles the cars had travelled.

Any one else carried out simillar tests?...

PowerLee · Unread post by **PowerLee** » 30 Nov 2005, 20:08

405 speedo always read 5 mph too fast through out the whole range

206 speedo reads 2 mph too fast through out the whole range

The 405 speedo is mechanical driven with a cable, The 206 is electrically driven.

Both check against the same GPS speedo on the Road Angel.

paranoid · Unread post by **paranoid** » 30 Nov 2005, 20:16

My 98 Xant is 5 mph fast at all speeds according to my tontom till it gets to 98 mph then it's spot on, but don't fast very often

jeremy · Unread post by **jeremy** » 30 Nov 2005, 21:03

Don't knock it if your employer pays you a mileage allowance!

Homer · Unread post by **Homer** » 30 Nov 2005, 21:16

My 807 is close to spot on but the works Mondeo reads way over.

In fact, if the mondeo's speedo is reading 85mph the actual speed is below the prosecution threshold by a good margin.

Very useful to know that I can safely pass a speed camera in a 50 zone with 60 on the clock.

bxbodger · Unread post by **bxbodger** » 30 Nov 2005, 22:56

Very useful to know that I can safely pass a speed camera in a 50 zone with 60 on the clock.

Put a new set of tyres on and you could soon end up paying for the chief constables christmas dinner........

Homer · Unread post by **Homer** » 01 Dec 2005, 20:08

bxbodger wrote:
Very useful to know that I can safely pass a speed camera in a 50 zone with 60 on the clock.

Put a new set of tyres on and you could soon end up paying for the chief constables christmas dinner........

In a strange way I relish the challenge.

np · Unread post by np » 02 Dec 2005, 00:48

I have a GPS Road Angel in my car.Up to about 60 mph,the road angel is about 1 mph behind.Then 70 mph in the car,R/A says 68,80 in the car,R/A says 77,90 in the car,R/A says 86,& 100

in the car,the road angel says i`m doing 95 mph.

Unread post by **DHallworth** » 02 Dec 2005, 14:25

i use one of the tomtom sat nav thingies, and i have noticed the same thing on the 306, can't quite remember how much in front it is, but its not right all the way through the speeds.

David.

citroenzx · Unread post by **citroenzx** » 13 Dec 2005, 09:30

I suppose you could then ask the question "how accurate is the speed on your gps device?"

allmond · Unread post by **allmond** » 13 Dec 2005, 18:06

Or, you could find a motorway, cruise at 120 kph (slightly illegal) and time yourself over two kilometres based on the posts at the side of the road.

Jamie

JohnD · Unread post by **JohnD** » 13 Dec 2005, 20:03

The mileometer on my Xantia recorded 5% more that the miles covered. That's according to Microsoft Autoroute. Also the recorded speed was 5% more than the speed calculated from the mph/1000rpm figure.

James.UK · Unread post by **James.UK** » 14 Dec 2005, 02:23

Aha!

That explains why I have had so many speeding fines over the years! I knew it couldn't possibly be due to my driving.. *looks angelic*

lol.

Homer · Unread post by **Homer** » 14 Dec 2005, 20:42

allmond wrote:Or, you could find a motorway, cruise at 120 kph (slightly illegal) and time yourself over two kilometres based on the posts at the side of the road.

The posts at the side of British motorways are less accurate than our speedometers.

As fo rthe accuracy of GPS, at a steady speed it is as accurate as you can get. Shame we can't say the same about speed cameras.