Bakertime, there's an explanation for why your mileage is better in 4th gear than 5th for the same speeds and same road grade.

If you scroll down in this page:

http://www.tc.gc.ca/programs/environ...nal_Report.htm

You'll see some pictures with a bunch of concentric rings, the name of the graph is "Engine operation during city driving cycle". Those rings mark off the efficiency levels, with a smaller number meaning more efficient engine operation. On the y-axis is your load--how much resistance your engine has to work against. On the x-axis is your rpm. If you'll notice, these are rings and not U's. What that means is that there's an upper-load limit for your efficiency at a certain rpm.

When you use a smaller and smaller gear, you're increasing the load on your car so that one engine cycle turns the wheels more than it did with a larger gear. The situation might change for you if you were driving downhill. 5th gear, in that case, *could* be more efficient, depending on the grade of the hill.

Kwestoff, the diagram in the above link can also explain to you why you can't just slap on a tiny gear and expect better results. You could probably do so and get fuel economy improvements for your driving situation, but it looks like this isn't a popular modification and the optimal values are unknown. There are other tricks you can do to improve your economy. Take out your passenger side seat, for one, and drive with half a tank of gas or less. Each one of those should take around 40 lbs off your car's weight. They'll also be much easier to do than pulling your transmission off. You can also buy a lighter weight battery and swap that with your current one. If you combine all of these tricks, you could probably reduce your car's weight by about 90-100 lbs.

There's some driving tricks you can also use to improve your mileage, described here:

http://www.cleanmpg.com/forums/showthread.php?t=1510

I do some "hypermiling", but it's not for everyone. Some of the small tricks (inflating your tires to the tire rating instead of your vehicle sidewall rating) will also improve your fuel economy without requiring any additional driving changes.

Um...some other tricks not listed on that site that'll improve your economy: switching to a lighter weight oil to reduce engine pumping losses (ie instead of 10w-30, use 0w-20 synthetic) and adding body modifications to your car to decrease aero drag (rear tire well covers, such as the ones on the honda insight).

My advice is that if you want to improve your economy, there's many easier ways to do it without pulling your transmission off your car. These other methods are also more popular, so you'll find more info on how to do them.

 

The least expensive way IMHO would be to go with a taller tire. This would reduce your RPMs at highway speeds, but the larger diameter tire will slowdown your speedometer and odometer (so you technically wouldn't see the increase since the odometer is how you calculate MPG). This would also make you slower "off the line" and MPG to get to hwy speed would probably decrease...

 

Larger tires will definately affect mpg on take off.

There really isn't no need to worry about the rpm's, it is natural for honda's to run at higher rpm than other vehicles. That is the way the cars are designed.

Besides the cost of changing your gearswouldn't be worth the results.

 

my 07 cruises around 3k roughly more like 2500, but its a newer car so that might make a difference in how honda has changed since then.

 

While indeed it would seem that larger tires would increase MPG, the fact that the car would now be higher off the ground allowing more air to go under the car, would tend to reduce the overall MPG. A better idea might be to use narrower tires (i.e. 155 width) at higher than normal pressure to reduce road resistance and not add to air drag. Adding a lower air dam to the front would also help, but my '06 Sedanscrapes plenty of times now when going into parking lots and pulling up to tire stops.

I just read this technical article on the gen8 R18A engine that is a must read for all gen8 owners. It turns out we have a reverse i-VTEC engine. When the VTEC kicks in it is at cruising speeds to save gas and not engaged while reving up no matter how high you rev. Indeed if you are an agressive driver you may never use your VTEC capabilities. This is the opposite of other VTEC engines.

http://asia.vtec.net/Engines/RiVTEC/index.html

Here is a quote from that article:

 

Thanks for all theinput so far. It appears as if changing the gear ratio in the tranny or differential would be a major undertaking, and anyreduction in engine wear would be questionable. I guess I'll shelve this idea for now. If I ever have to pull the engine I might makea small change in the ratio just to see what effect it might have. Hopefully I won't get the opportunity.