It makes a huge difference actually. The Civic Si has a super short 6th gear and it can do high 30s cruising. Typically cruising gear places the engine in the 20% full BMEP range, where efficiency is horrendous. Just by changing the gear, you can typically reduce fuel consumption 30% under steady state cruise, although it reduces the extra power available.
If the Civic Si was geared a bit longer, it could do >>40mpg. Any modern, small car is capable of 45mpg on the highway or better if the highest gear is at the right ratio. Engine mechanical efficiency these days is pretty high, combustion efficiency is good, so what's left is pumping loss due to suboptimal operating conditions.
FYI, the brochure released says auto is 4.100 final drive, 0.582 6th gear, while manual is 3.727 final drive, 0.767 6th gear. That puts 60mph at around 2350rpm for the manual, 1960-70 rpm for the auto (quite remarkable, my mom's 3.5L Honda Pilot has this cruising rpm!). The auto would probably be somewhere around 80-90% peak efficiency, while the manual would be at 70% of peak efficiency or so. The mpgs an automaker can pick up by making 5th/6th gears taller is by far the easiest mpg they can pick up, as it costs no money whatsoever.
See my posts in this thread.
http://www.ft86club.com/forums/showp...8&postcount=60
The only downside is you can't be lazy and expect great passing power in 6th. IMO 2000rpm at 60 is a good gearing, more than enough torque to go up the steepest grades, efficiency nearing 10% of peak.