Technically 4th and 5th are overdrive (transmission output rotating faster than engine). If there were a sixth gear, the RPMs would be lower still at a given speed, and acceleration would suffer. Is that what you want? I ask because sometimes people say "overdrive" when they really mean "passing gear," which is in fact a lower gear. In that case, just hold it at 4th on the highway.
Or do you wish for lower RPMs to save fuel? 3,500 RPM really isn't high for a modern 4-cylinder engine. Is your mileage significantly lower than the EPA estimates on the sticker?
OP, you need to fill out your profile so we know what could be going on.
What do you mean by 5sp? It's not safe to assume manual transmission b/c Honda gave the 07+ a 5sp auto transmission. Since I drive the auto, I'm seeing your post and wondering if you have the car locked in d3, that'd kick the RPMs pretty high on the highway. However, that would be crazy talk if you've got an MT.
I was guessing that the OP drove an AT because he said it was "new" (as opposed to "new to me," presumably), thus a 2010. However looking at other posts it seems he has an MT after all, despite Honda's assertion that MT isn't offered for the current model year.
What I said above still applies, however: 5th gear is overdrive; 3,500 RPM isn't high for this type of engine; and it's still unclear whether the complaint is insufficient power at speed or inadequate fuel economy.
70 MPH, '08 SC w/ AT, turning approx. 2,300 RPM
Now, for the record, that RPM is after it had gotten to speed and began to cruise - on a flat section of roadway. When it began to crest and needed a bit of power, it was about 3,000 RPM after the downshift!
I just replaced my 06 EXP with a 2010 EX and the 06 @ 70mph was always around 3000 rpm and I drove this one 88,000 miles and gave to my grandson to go to college. bought the 2010 this week and at 70 mph engine rpm is 2200 and I have already drove this new on 1700 miles in 4 days and I really like the changes that honda made from a 4 speed auto to the 5 speed auto I drive on all type of highways