Increasing speedo accuracy

Xaccers · Post by **Xaccers** » 12 Jul 2007, 12:08

Is it possible?
When my sat nav is showing me as doing 60mph, my 2.1TD's speedo is saying 70.
Is there anyway to improve accuracy at that end?
I've never had a car so far out before.

Post by **RichardW** » 12 Jul 2007, 12:31

I don't know what happened to the later electronic speedos - my 95 TD was pretty close, and my wife's 94 ZX (Cable) is pretty well spot on. My 97 TD is wildy optimistic though - much the same as yours (maybe this is why people get 50mpg out of a 1.9TD Xantia - they are driving at 50mph indicated, but it's actually about 42 or something)! I just ignore the speedo and drive off the rev counter, where gears 1 - 5 = 10,20,30,40,50 mph at 2000 rpm pretty well spot on - and 3000 rpm = 75mph, which is easy to follow on the motorway, as it's needle sticking straight up - I think the ML5 box is longer geared though, so might not be so easy! In theory I guess you could get it recalibrated, depends on whether you can change the programming of the PCB or not. Or get Jim to build an attenuator to change the signal output

AndersDK · Post by **AndersDK** » 12 Jul 2007, 15:27

Most simple bodge : Larger tyre diameter

Works perfect on my oldie BX'es. Only snag is, that the warning devices now warns me at correct speedo readings. No more "safety range" up speed from the speedo readings. You'll very easy forget that ...

Post by **elma** » 12 Jul 2007, 15:37

The gps isn't necessalary accurate either. Mine says I'm doing 85mph for 2 hours to do a 200mile journey. It can only be correct if you travel at a constant speed in a straight line.

The car speedo must be 5mph high because of eu legislation.

tricky ricky · Post by **tricky ricky** » 12 Jul 2007, 17:06

My Xantia reads approx 5Mph high across the board when referenced against the sat nav. The BX is slightly high at speeds up to about 40 but its apparent accuracy increases above that

lolingram · Post by **lolingram** » 12 Jul 2007, 17:15

It is not only possible, but spot on. What wheels/tyres are you running on?

Is it possible?
When my sat nav is showing me as doing 60mph, my 2.1TD's speedo is saying 70.
Is there anyway to improve accuracy at that end?
I've never had a car so far out before.

Stempy · Post by **Stempy** » 12 Jul 2007, 18:38

Don't know about the Xantia speedo but I did a lot of experimenting with an electronic speedo from a BMW motorcycle which was wildly optimistic and found that it was possible to recalibrate it by adding and removing the jumper links on the pcb. These links were obviously for calibration purposes and grounded out a number of pins on the drive chip. I got it spot on by trial and error, maybe I happened upon 'police spec' settings.

lolingram · Post by **lolingram** » 12 Jul 2007, 19:44

A speedo can overread your true speed by up to 10% but cannot under read it. So if your speedo is indicating exactly 30mph your true speed can be anything from a shade over 27mph up to 30mph. Conversely if you are doing a true 30mph your speedo can legally indicate anything between 30 and 33mph.

There is no current EU legislation that insists on speedos being +10%! SatNAVs will give TRUE instantaneous speeds with a sample rate of around 5 seconds...

The gps isn't necessalary accurate either. Mine says I'm doing 85mph for 2 hours to do a 200mile journey. It can only be correct if you travel at a constant speed in a straight line.

The car speedo must be 5mph high because of eu legislation.

Xaccers · Post by **Xaccers** » 12 Jul 2007, 23:37

lolingram wrote:It is not only possible, but spot on. What wheels/tyres are you running on?

205/60 on Activa alloys.