I've been driving my 1994 Chevy C1500 W/T now since early 2012. I just rolled 417000 miles with the original 4.3L V-6. I'm on my 3rd Getrag NV3500 5-speed transmission, the one it came with, the used one I put in it, and finally, the rebuilt that's still going strong. I've put a set of Good Years about 40000 miles ago, P235 75 R15. Since day one, the speedometer has been off. At slower speeds, it's pretty close but the faster I go, the farther off it reads. As an example, speedo reads 40 mph, I'm going 35 mph. When speedo reads 75 mph, I'm going 55 mph. Once the speedo is buried, I'm just keeping up. Anyone have any ideas or suggestions?