I filled up today and the pickup is getting 12.7 MPG when it was getting 16 to 18 MPG.
I am sure this is due to many factors 305/4L60E taken out 350(Rebuilt)/4L80E put in it's place, the slight change on the speed o use to read 70 when you were doing 68 but with the swap it is dead on 70 = 70, could the cat starting to plug rob some MPG? or would there be other symptoms from the cat plugging or starting to fail.
It just hit 182,000 miles.

 

If the cat was starting to plug, you might notice a loss in power, often the engine won't rev above 4k and will start to ping and misfire.