So when I bought my truck the guy told me he had a new rear-end put under it due to the old one makin a lot of clunking noises. I noticed in the dash it has the G80 RPO code. How can I check and see if the rear-end that's under it now is also a G80? and what would cause the speedometer to be off by about 6-7mph? This is on a '97 ECSB 4wd. 5.7L 4L60E. 265/75/R16 and by the RPO codes it supposedly has 3.42 gears.