Like I said above, power mods will not affect cruising RPM!!! When you're accelerating yes, you will make more power and thus may take less RPM to reach speed. However, at speed (cruising and just maintaining speed on level ground), RPMs are only a function of tire size and gearing.
Think of it this way:
You're tires are turning the speed they need to go at 70 mph.
The tires turn the axlehalves to turn the ring gear.
The ring gear turns the driveshaft 4.10 (or 3.73, etc) times for every rotation of the axles.
The tranny is in it's 1:1 gear, so the flywheel is turning at the same rate as the driveshaft.
Thus, at a cruising speed RPM is dependent on the rotational speed of the rest of the drivetrain. Yes, when you need to accel to pass your RPMs are going to go up and the more power you have the less the motor needs to increase RPMs to apply the necessary force.
Make sense now?
The reason I say that you compare highway RPMs in your 1:1 gear is that 1) it is an industry standard and 2) I've found that in OD RPMs are not always exactly where they should be due to OD and torque converter wear and differences.
