about 3000 rpms. i usually cruise on the highway at 75 at like 2600 rpms. the motor seems relaxed at highway speeds. think about it you have 5th gear , its not like your driving a 4 speed, you have overdrive. i'm used to munci 4 speeds where you are screaming at 55 in 4th with 4:55 gears. dont fear the gear, its all good get the 4:10's you wont regret it
high enough to feel uncomfortable after 5 minutes ... it sounds too high for me when im cruising at 80mph and i have 3.90's.... im probably at around 2400-2500rpm i think
Anywho...about the rpm, don't worry about gas milage, it's really not that big of a difference: at max, 2mpg.
Here's how you should determine what gears to get: If you plan on getting a power adder, then go with 3.73s. If you plan on staying N/A, get 4.10s. 4.10s are just too steep for most power adders IN MY OPINION.