I accidentally overlooked the first calculator that allowed for selecting transmission type. I used the one below that for my numbers. I re-did my numbers with the selection of automatic transmission and got the same results you posted...which make it look even worse.

 

I want to update this topic with new info. Someone in here suggested my 29 year old factory tachometer might be off. So I hooked up my timing light (w/RPM display) and took the truck for test drive. It revealed my factory tachometer was running high.

.................Factory Tach.................Timing Light
At 55mph = ~2350 rpm ........................ 2260
At 60mph = ~2700 rpm ........................ 2470
At 65mph = ~2950 rpm ........................ 2690

The numbers using the timing light are much closer to the previously mentioned online RPM calculator.

I'm starting to feel I don't have much to worry about, the rpms
are normal-ish.

 

the slip numbers look about right, remember at higher speeds with our trucks aero the drag goes way up causing more slip so at lower speeds less slip. A normal converter never fully achieves 1to1 operation.

 

Hello mine is also close to your posted results still I'm unsure of torque converter being in lockup as anything over 55 feels like I'm wasting gas lol
I also have c6 3spd with over drive

 

There is no lockup in a C6. And you're responding to a post that is almost 5 years old since the last post.

 

and the C6 is 1:1 in third gear. There is no overdrive gear.