Mines doing the exact same, got the sled with 62 weights and it pulled 8400, put in 64s and it was 8200, most guys around here are running 66s. I didnt pay much attention after that, blew a belt replaced it with a ultimax, went to the mountains with 58s and came back put the 64s in and noticed 7800rpm! then i put in 62s and only 7850, checked my belt and it was 1.75``deflection so i tightened it and gained a bit more but not 8400...... i know the belt is different but should not be 500 rpm different. I think the springs are stock from 08 and it has almost 5000 miles on it, its mainly driven around -10 in sask. So im thinking my springs got heated up enough in the mountains that they lost some tension. I dont really want to put stock springs in, i would like a better set up, possibly a helix so i might go with the carls kit or if anyone knows a good set up for 1500 ft that i can just change my weights for 8000 ft that would be great.