Converting Speedo to GPS?

[n0m4d]

One of the things (among many) that drives me nuts about my car is my speedo is way off due to varius factors, mostly wrong tire size. I was trying to figure out how to correct this, read some threads about tweaking the cluster, etc.

Then I came across this:

http://www.jegs.com/i/JEGS+Performance+Products/555/41612/10002/-1?CAWELAID=1710606761&catargetid=1784155633&&cagpspn=pla&gclid=CO2FlcvF47YCFeN_QgodLSoAjw

Its basically a GPS based speed sender. It specifies that it works with "All speedos that accept 4000, 8000, or 16000 pulses per mile". No clue if that is what our car sends.

In my head (which is often not a good place for ideas to live) I'm thinking I could replace the speed sensor in the diff with this and get an accurate speed reading regardless of tire size.

Crazy or brilliant? (i do know gps speed readings may be a little laggy).

Edit: This one is more expensive but seems to be not limited to any specific pulses: http://www.autometer.com/cat_accesso...l.aspx?vid=234

[Gregs///M]

There is an easy procedure for Motometer speedos to adjust speedo calibration factory. VDO is a little more complex requiring replacement of resistor with a different resistor.

Try searching how to adjust the speed calibration factor for our speedo's.

[n0m4d]

This is the thread that I had found previously: http://www.e30tech.com/forum/showthread.php?t=73541

I wasn't in the mood to take my cluster apart I guess. The Jegs unit would likely not work. The Autometer unit would just require power, ground and access to the speed pulse wire, all of which are under the rear seat. The antenna would sit nice on the rear deck. This would in theory give a accurate speed regardless of tire size so switching them around wouldn't mess with the speedometer or economy gauges. Tunnels would be an issue but I don't drive thru them like...ever.

Once I get my tranny fixed I'll try it, if it works I'll post a how to.