Hey all, my speedometer is off by 4 MPH (too fast) at 60MPH, confirmed with GPS and a chase vehicle. I have stock rims (15x8) and tires 31x10.5. This is 87 V1500,700R4, NP208, w/GU6(3.42), 53K original miles; I bought this truck a few months ago after it sat mostly since '05, and I've done a...