Speedo reading incorrectly - help!

[tricky]

The speedo on my j-spec seems to be out. Its got an mph dial, but seems to be showing a higher speed than im actually doing. Ive checked it against a tomtom and a garmin sat nav, one says its 7mph out, the other 8mph. So if the clock says im doing 50mph, im actually doing 43mph. (helpfull for not getting nicked) Are the sat nav readouts reliable? Can someone help me out here, is it still measuring kph or something else. Anyone know how i can solve this?

[Martin F]

Sat nav readouts are pretty accurate.

What size wheels do you have fitted to the car, as these can make a difference to the speedo reading.

If you have a Thor DSC, these are apparently adjustable to compensate for rolling circumference (wheel\tyre size) differences.

[tricky]

The wheels are 17s with 225 45 tyres. I did a check with a tyre calculator thing that compares original to new sizes and it said it would make a differance of 0.36 mph to the readout.

I havnt a clue if ive got a thor dsc. I dont know whats been done to convert it. Has it? or has it just got a mph dial face.

[dwayne]

i think i have the same problem mate. i got my brother to drive in front of me and kept the same speed and that shown i had about the same difference as you and i also have the same tyres as you.
if you find out what it is and how to fix it let me know and i'll do the same.
good luck!

[Martin F]

There should be a speedo converter and it's usually placed on the back of the speedo\tacho assembly.

A lot of the importers wire them up incorrectly and it can cause all sorts of problems with power steering, auto boxes, active spoilers, etc. Now may be a good time for you to investigate further and find out what type you have and how it has been wired in.

[dwayne]

cheers mate, do you have a link speedo removal so i can take a look

[tricky]

I wanted to take the dash out to investigate the stereo anyway, so ill check that out Martin. Do i have to remove anything from the dash ie. switches before i can pull it out. Ive had the top bit out with the warning lights so i now theres a couple of screws, but the main dash still seemed tight with these removed. Can you tell me what to remove to get it out.

[Guyver]

Try this link about getting the dash out. Can be done in about 5 mins easy when you know how.

http://www.internetwork.org.uk/Dash%20Conversion/dashrem.htm

As for the speedo since I got my Tomtom every car I've used it in the speedo has read over by about 5mph at 70. They get more inaccurate the faster you go but always seem to read fast (a good thing with all those cameras about).

[tricky]

Thanks for that Guyver, very helpful, i'll give it try later on.

[tricky]

Ive had the dash out and theres nothing on the back of the speedo, its just the original wiring loom plugged in. So its obviously reading in kph still, just with a mph dial face. Looks like ill have to invest in a thor dsc or similar.