Do you have the specification card on the cam that's in your engine? The most economical cruise RPM will be just slightly above the torque peak. If you get it lugged down too far, the power valve in the carb will open up and kill your gas mileage, plus the acceleration will suck until you struggle up to the RPM where the engine starts making usable torque. Try running a vacuum gauge and see which size tire allows you to accelerate in top gear with the least amount of vacuum drop.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!