I have a 2004 Silverado, 4.8L V8 and 5-speed tranny. Original 4.3 V6 was swapped out. The truck runs and drives fine but ... in 5th gear and cruising at 50 mph the tack says 2000 RPM! (at 65 mpg the tack says 2500 RPM). Tires are stock 235 x 75 R16s. The only way I understand this happening is if the rear diff ratio is a 4.56. The glove compartment code says G80 and a GU6 (for a 3.42).... but the rear diff may have been changed by previous owners. No tags on 10-bolt rear diff, but numbers on casting are 8857, 4017 and NC18. I drove a similar Silverado with a V6 and 5-speed, and at 65 mph the tack read only 2000 rpm. Breaking open the diff cover is my last resort. When I rotate one tire clockwise the opposite rotates in the opposite direction. If I hold either tire still, the other tire still rotates. The driveshaft (with tranny in neutral) completes approx 17 rotations for ten wheel rotations. Nothing makes sense here (to me). Can anyone help me understand why the RPMs are so high? Thanks in advance.