Quote Originally Posted by Squibcakes View Post
A cheaper option would be to use a hall sensor mounted on your drive shaft and have the pic count the rpm. Convert that to mph and voila! You have your speed.
(If you want to get even more fancy, you could then hook the pic into your throttle and hey presto!, you've just built a cruise control for your car. Lets see the GPS do that!!)

Good luck.

Squib
I would have considered that but I've already got something similar, its my speedometer, my reason for wanting to do the above is the accuracy of speed derived from GPS signals, where using sensors on the driveshaft or anywhere else will never be anywhere near accurate due to tyyre sizes, and even the tyre pressure dropping would change the speed reading's accuracy thats why I ruled out sensors on the cars mechanical bits