I've highway driven with 4.30, 3.91, 3.55, 3.23... and loooong time ago 2.76. The car was typically going to an autocross, or road race track (and occasional car show or cruising).

Currently I'm running the 3.23.. excellent gear for hwy cruising and HHPDE/HSAX and pylon AX... you'll learn the gears (1-2-3, 727) to run whatever event your involved in.

With the 3.23 I can run 1st gear in most pylon autocross events; briefly 2nd, then back to 1st.

With the 3.55 I found it necessary to be 2nd gear more often (slower AX times).

The 3.91 was great; the 4.30 was also great (although too much 1st gear tire spinning and used 2nd more often).

At track lapping events (HPDE/HSAX) the 3.23 is good.. lots of 2nd gear then 3rd gear. The 3.55 needed 3rd gear more often but lost top end mph. The 3.91/4.3 track lapping required 3rd gear dominantly and limited top end mph.

I also have a 2.9 ring/pinion that I'm eventually going to try; I know the 2.9 will reduce the hwy rpms by about 450-500 depending on typical hwy cruising speed (~65-75).

My thoughts with the 2.9 is longer stretches in 2nd and higher mph with 3rd gear... as well as dropping the hwy cruising rpms by 450-500 for long drives (short drives doesn't matter as much).

My engine (340-6) likes to cruise at ~3200-3500 ~65/75.

Look up one the available rpm/mph charts on the web and you can play with that to determine what you'll like. Tire diameters matter; as well as your transmission gears (727 vs stick).