Do you get your 3rd gear if you go 55 instead of 65? Is 10mph gain worth redoing the rear end or chip tuning the motor?
Get one of those bluetooh scan tools from Amazon for $15 and buy the Torque app on the App Store for Android or Apple. Of the various things it can tell you, one thing is "Engine Load %"; if you're on the flat at 65 and your engine load is in the high percentage...
Something else to double check, at least around here, a lot of "flats" are actually uphill grades, it just doesn't look like it because there's no good reference (like water) - if the landscape slopes at the same degree as the road, it looks flat, and those can go on for tens of miles.