I was reading a post about the effects of RPM on gas mileage, It was from 2007 so I couldn't reply to it. RPM does make a difference on gas mileage when it comes to overdrives. I had an '82 Buick Riviera with a small block engine with an overdrive and a quadrajet that got 26 MPG on the freeway and about 21 in the city. I now have two 1968 Chevys with carburated 327s that only get about 17 MPG. One had a 2.73 rear end and the other a 3.08. The RPM on the Riviera going 70 MPH was only 1500. The '68 with the 2.73 was about 2400.