Your engine may require more RPM to reach max efficiency compared to stock. If the engine is built to make more power it probably will make that power with higher RPM's, which could better match the cam timing. I don't think lower RPM's always mean greater efficiency, or lower fuel consumption. I've always wondered if there was a simple formulae to calculate what the best RPM would be for cruise efficiency. I think it needs to be someone much smarter than me to know that.
Crane Cams do list a cruise RPM on their cams where their cams will operate at best efficiency on the highway. The roller cam I am looking at states 1,500 - 5,500 rpm power band, 6,000 rpm redline, and 2,400 to 3,000 rpm cruise.
So long as I maintain 2,400 - 3,000 rpm while on the highway I will be in the most efficient rpm range for the given camshaft based off the manufacturer.
Problem is for me once I move highway speeds which is what all my driving will be being out in the country has speed limits ranging from 65 to 75 mph. People do 75 to 90 mph so I tried to set up my axle ratio to try and give me the ability to do 80 mph at or under 3,000 rpm.
With a 3.00:1 axle ratio 2,400 rpm with 3% to 5% slip would be 69 to 70 MPH with 3,000 rpm with 3% to 5% slip would be 86 to 88 MPH. If I go with the 3.25:1 axle ratio 2,400 rpm with 3% to 5% slip would be 64 - 65 mph and 3,000 rpm with 3% to 5% slip would be 80 - 81 MPH.
So for me 3.25 would give me the most highway speed range in that narrow 2,400 - 3,000 rpm cruise range but 3.00:1 axle ratio with taller tires would put me closer to the OEM axle ratio of 2.75. The 3.00:1 would give me a 2.72:1 axle ratio when accounting for my taller than stock tires, and 3.25:1 would give me 2.95:1.
So either way I go both axle ratios will work for me, but the 3.00:1 would allow me to run 80 mph under 3,000 rpm but at the same time I am outside of the cruise rpm at speeds below 70 mph. The 3.25 would give me better acceleration and would allow me to change my speedometer gear in my C6 to make my speedometer accurate as well as allowing me to run between 65 MPH and 75 MPH and maintain cruise rpm. I just don't know how well a small 306 build would like spinning 3,000 rpm for 3 or 4 hours at a time with Scorpion roller rockers. Its kinda why I keep entertaining the idea of the 3.00:1.
But I do plan on going through the motions to get this 306 as efficient as possible. I built a 02 calibrator in a box using a air/fuel ratio gauge to allow me to read air/fuel ratio at different conditions to help me jet and tune the carb to be slightly lean for no load conditions and be rich on load applications for max torque production. Its also why I got the quick change vacuum secondary kit to convert my 600 cfm summit 4V to have the holley quick change ability so I can custom tailor the secondaries to my specific truck.
Its why I am trying to plan my future steps out as well such as transmission and rear axle which are the next two areas after I finish my engine build. I want to ensure all areas are going to complement and work with one another. Just like for instance I know being a daily driver it will have to be capable of doing 75 to 80 mph to be capable of running on the highways just because other people are asses and will ride your bumper for not going fast enough trying to push you.