I'm in the process of LS swapping a 1984 Chevrolet C20 with 5.3L(E38)/4L70(T42) out of a 2008 Silverado. My first issue is I've read where people have had issues with the truck OS and the 4K VSS output at pin 57. I have an electric speedo out of a 90's suburban that is compatible with the output however all it does is peg out every so often, it doesn't actually work. I've also read where changing to the vette operating system out of a compatible year could possibly fix this issue. My second concern is the VSS does not read on the computer until 15 or so mph. The truck has 34" tires with 4.56 gears, and I've tried several set ups to see if the calibration is just that far off. The tune on the ecm originally was for 31.02" tires and 3.42 gears. The tcm was originally set up with 6" tires and 3.73 gears. I've tried putting actually numbers in the tcm but it drove horribly, the best I've had it so far is by adjusting the gear and tire size up on a percentage. I've checked plugs for a loose wire or connection and I haven't found any. Could internal damage in the tail shaft cause the delayed reading(too much slack in a bearing?) I'll post my current tunes, all I have modified is the tire size and gear, the rest was done by my harness builder. I'm going to put it on a dyno for the final tune, but the delayed speed reading has me concerned that I might need to fix something before I try to get the final tune.