Answer:

Post by Stuart on madabout:

This website explains the electronic calibration method:
http://www.caigauge.com/speedofitold.htm

Basically on the E30 you use Wheel circumference and the following formula.
Number of magnets x half shaft or wheel revolutions per mile or per kilometre = calibration number.

if you use this site it will give you tyre circumference for your vehicle.
http://www.miata.net/garage/tirecalc.html

I have 225/50 x 17 tyres on the back so circ = 81.2 inches. There are 63360 inches in a mile so 63360/81.2 = 780.3 revolutions per mile.

Now on the standard BMW sensor there is 9 magnets but check yours with a multimeter.

So back to the formula:
Number of magnets x wheel revolutions per mile = calibration number
9 x 780.3 = 7023

This is your calibration number 7023

Now you need to convert it using this table:
http://www.caigauge.com/speedocalib.htm

Not all numbers listed so go for the closest you can find
7031 = 0100100010

0100100010 this is your dip switch settings.

Hope this helps i did mine this way and checked it against TomTom GPS speed on the way to the SVA centre and i was on average reading 1-2% faster on the speedo this was comfirmed during the test.

Stuart