Sometimes my 2004 Chevy Tahoe Z71 will be at 2000 rpms @ 70mph and sometimes around 2150-2200 rpms at 70mph. Is there anything wrong with my transmission? The wheels are 265/65/18. Sometimes the rpms run lower and sometimes then tend to run about 100-200 higher through all the gears. Is this normal or something go bad with the transmission? Has 157k miles but shifts through the gears smoothly and no slipping or hesitation of any kind.