Hi

My speedo over reads by quite a lot, for instance in a 30mph it will read 40ish,

I know i can get it recalibrated, and will do once all my other mods are done, a i will be changing the wheels and tyres,
also possibly the gearbox,

Firstly can i adjust anything in the speedo? to get it nearer the correct speed myself? or is it really a send off and wait job

also, by downsizing my tyres to a smaller circumfrence wil do what to my reading? as that could be a factor in wheel and tyre choice

I currently run 205/60/13 and was thinking more on the 185/55/13

Steve

well the profiles would be as follows:

205s = 123mm
185s = 102mm

Your speed is calculated from the gearbox as you have the mechanical drive, so the only easy real way to adjust this is to change the tyres. A smaller profile would combat your problems

Cheers Andy, perhaps some more wheel spinning in your close may sort my speedo out !!

Steve, i think i've got that wrong actually.
The speedo will be setup to whatever the car's tyres were originally... So assuming it's the drive from an escort for example, it will be configured to standard escort wheels/tyres, e.g. 185/60/14 or similar.
Obviously as you have 13s your overall radius will be less, and so a smaller tyre could actually make it worse.

Effectively the speedo will think that as you do one revolution, your wheel would have travelled a certain distance, so the bigger the wheels, you should get under-read, and the smaller the wheels you will get over-read

How have you checked it anyway, with a gps?

I used my Tomtom, when i went to westerham a while back, and i was over reading on the car speedo
at 80ish on the tom tom, my speedo was up near 110, and there is no way my 7 will do that!

Also, my speedo is wrong, as it was set up for my other gearbox, that was down on acceleration, but good on higher speed cruising,

also, 2 ! my speedo/cable/ etc etc are not escort, there unknown, and up to when i changed my gearbox was all correct

[Edited on 30/8/11 by steve m]

just found this mate, have a read, might be helpful!
However, if you're planning on changing the tyres anyway, i'd probably do that first....

Weres the link M8

oops!
http://www.rodandcustommagazine.com/techarti cles/0801rc_mechanica_speedometer_calibration/index.html

Calibrating a speedo depends on the make and technology. If its a digital unit then that can be quite simple (and the method will be in the instruction manual).

Getting a conventional speedo calibrated will require the gear wheels internally to be changed which is a send off job. Speedycables are probably the best known people in the UK, but at £55+VAT+carriage it would be cheaper and easier to get an Acewell speedo from Russbost if the styling is to your taste.

http://www.speedycables.com/calibration.html

Changing wheels and tyres sounds a bit extreme and even more expensive.