MadaboutKitcars

I'm getting a bit ahead of myself, but i'm thinking about Speedo calibration.

With potentially only one drive before the SVA test, how do you get some confidence that the speedo is working correctly?

So i'm thinking....... if you put the back of the car jacked up securely on axle stands, place the gearbox into 4th gear (1.00 ratio) and run the engine at 2000 rpm. That should give 'xx' mph on the speedo. (Speedo sensor is in the differential)

The question is...... how do you calculate 'xx' ?
It must be something to do with the diff. ratio and the circumference of the rear wheels.

Has anyone got the formula or indeed what speed a 1989 BMW 320i goes at say 1000, 2000 and 3000rpm in 4th gear?

Peter

Peter

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

That's terrific, thanks for the comprehensive reply.

Got that sportster painted yet? DO post an update if you have.

Peter

Thanks Stuart big help there I hope you don't mind, I've put this in the FAQ on my site

speedo/wheel size

thanks for that info, this should make thing easier.Stuart I note the tyre size you have used.What wheels, offset and spacers did you opt for? Cheers