1991 GMC Sonoma

Computer problem
1991 GMC Sonoma 6 cyl Four Wheel Drive Automatic 141000 miles

My pickup's digital speedometer jumps all over the place from time to time, really fast, from 0 to 185mph to 30mph and so on, and then will settle down for a while, but it usually shows only half the speed I'm really going. It even jumped around when I was sitting still in my driveway. There is no check engine warning light. How do I go about finding the speed sensor and replacing it? Engine is 4.3 V6.
February 18, 2008.

Check out the related content below while we wait for the question to be answered by a professional mechanic.