My dad owns a 95 GMC Yukon 2 door 4x4, just like the one I drive and em tuning but he bought his new from the showroom floor and got a rebuilt 350 two years ago but since only my brother drove it it has probably less than 30k miles on it. It always seemed to pull harder in 1st gear than my yukon even after I tuned but I could still beat him and outrun him on the short run and long run. Well I finally bothered datalogging his Yukon and found out that His knock sensor is not working so the ecm is not pulling any timing whatsoever and his temp is not rising about 60*C meaning that his thermostat is stuck open or missing.