I can see that you’re skeptical, but it really is basic physics. Yes, your mileage will drop if you lock it into 4th gear, but not by as much as you might think, and not simply because of the higher RPMs. Think of it this way: if the gearing were switched so that the RPMs went from 4000 to 2000 when traveling at 80mph your mileage would not double, as you might expect if it were simply a function of the RPMs.

Instead, it’s really a question of efficiency. At what RPMs does this engine most efficiently convert the energy in the fuel into horsepower? I don’t know this figure, but the Honda engineers definitely do. And the key is to use the gearing to keep the RPMs as close to this figure as is possible.

However, straying from this figure slightly--either by going above it or below it--is not going to make a huge difference in gas mileage. The OP is asking about mods to lower the RPMs to save fuel. I’m pretty confident that the costs involved in doing so would be higher than the savings you’d get from the slight increase in fuel efficiency wrought by bringing the RPMs closer to this ideal figure over the life of the car.

 

Although MPG was a question asked at the end, what I took away from the OP's question was more along the lines of "3500-4000 rpm being unbearable". What I read was cabin noise was high and that was the desire to drop the RPM's. I took the MPG as a side question asking if they changed (AFTER any modifications were done to drop the RPMs).

~SB

 

Actually it's slightly more complicated than that... But the easiest way to figure out where the car is most fuel efficient is simply monitoring spark advance, AFRs and IDC percentage with a scan tool as you cruise at different speeds.

And as specboy pointed out in an earlier post, the easiest way to bring down engine speed is just simply get a marginally taller wheel tire combination. 5-10mm more sidewall would bring you down 200-400rpm depending on tire temperature and pressure under extended cruising.

Dropping from 3500 down to ~3100rpm will make a big difference if VTEC disengages.

 

I bought the 2010 5MT because I missed the fun of a manual transmission. My 98 Accord is fun because of the Power, but the trany is horrible. It is going out again at 100K. I wish the fit had a 6th gear, but now I just crank the music up. The fun is better than the noise. Paddle shifters just are not the same.

 

Of course, if you change tire size, you need to recalibrate your speedo and odo or they are going to be off and your actual MPG will be different from what is being calculated.

Zach

 

My understanding is that that point of VTEC is to maintain high efficiency over a relatively wide range of RPMs. If so, then running at a somewhat higher RPM shouldn't lower efficiency, and so won't reduce gas mileage. I suspect that the only loss in efficiency you'll see from running at the higher RPMs will be from the slightly higher pumping losses. This does seem confirmed by the EPA mileage figures for the AT and the MT. If I recall correctly, the AT was rated about .5 MPG better than the MT at highway speeds.

As far as noise is concerned, I find that by far the greatest source of noise on the highway is the tires. Wind noise and engine noise are a distant second. So I personally don't see this as a good reason for spending lots of money to get taller gearing either.

 

Increasing tire size is the only realistic means of lowering rpm as all the others cost a lot more than trading your manual for automatic. Even then only a small increase in tire diameter will fit the tire in the fender well so you might lower rpm by 5%, not worth much either.

 

Agreed. my calculations were that a 1" increase in radius (which I don't think would fit in the wheel well) would drop the rpm's by 500 at 60ish mph.

JDM 6sp tranny swap or moving to an auto is the only real way to drop RPMs. If it's noise... Dynamat might do some good.

~SB

 

I'll never forget in high school a friend of mine was thinking along the same lines and put an automatic rear in a manual GTO, it would go about 80 in first gear and you really couldn't get out of second, lol