I also would be very interested in a lower rear ratio, or perhaps a 7th gear as previously mentioned.
Unless one is towing something, or going up a major hill then the engine will very likely have adequate cooling and oil movement, even at lower RPMs.
The reduced RPM will definitely reduce fuel consumption. The reason is that for a proper combustion burn you need a fairly precise fuel/air ratio. So if you are developing the same HP (ie same cruising speed) at lower RPMs then the pistons are sucking less air per minute. (Less piston strokes/minute due to lower RPM).
There fore at exactly the same air/fuel ratio then less air = less fuel.
Basic internal combustion physics.
As far as bought having bought an S2000 vs (say) a Toyota Corolla that statement says it all. The S2000 has a lot more going for it than just a screaming engine and wheel spin HP to make it a desireable car to drive.

 

You can swap out 6th for a higher one.. it's expensive. someone used to make one, but I think the kit has since disappeared.

this is a popular mod in old VWs.. I have a taller 5th for highway cruising in my 84 GTI. granted it kills the 1/4 mile time because it redlines 4th short of the finish, but that's a different story

 

Actually AP1s like mine run leaner at certain rpms. Mine actually runs leaner at 4500rpm than it does at 4000. If I cruise at 80mph I would get better mileage than I would going at 70. The point you made about the piston was correct, but ultimately the ECU decides how much fuel goes into the engine.

I have a wideband a/f ratio so I can constantly view my afr's and see at what speed I would get the best gas mileage. I also have a VAFC2 which shows me the throttle position so I can have another way to view what affects my mpg. And in my opinion all the money you would spend doing some crazy 7 gear transmission or installing a 3.90 final drive gear, youd be better off spending those couple grand on gas if your worried about mpg.

This thread was made because the OP doesnt believe an engine that can rev as high as 9,000 can handle being at 4500 rpm for a sustained amount of time. Im not sure if this is still a concern as many people have already said this is a non-issue.

 

Sigh... To begin with, my car redlines at 8k, not 9. If you had followed the thread past the first two sentences, you would know that ~5500rpm cruise speeds were discussed. You could fit the mechanical knowledge of the average car forum poster in a lunchbag, so forgive me if I take "many people's" opinions with a grain of salt. I did not ask whether or not the engine speeds that I cruise at are harmful to the motor, but simply how to reduce them. Since many of you had no actual information to offer, you twisted the post around to make comments.

 

Thanks for that link. I wanted to show my cousin how to calculate gas mileage, but it got confusing. That link helps out a lot.

 

AP2's can be revved up to 8,600 without any harm. With the bigger pistons at 8,600rpm theyre at the same speed as an AP1 piston at 9,000.

For you to be cruising at 5,500rpm in an AP2 you would need to be doing 100mph. If you want to spin lower than that you might want to try driving a little slower

At 80mph you should be just over 4,200ish and thats completely fine.

Everybody has already told you what you need to do to fix your problem. Im not sure why you choose to keep ignoring it. They sell 3.90 final drive gears. You can spend $1000+ on this setup. This will lower your rpm at 80mph from 4,200ish to about 4,000. And at 100mph youd see 5,000rpm down from 5,500.

You could also get some taller tires that would probably knock off another 1 or 2 hundred rpms. Honestly theres nothing you can do to DRASTICALLY reduce the rpms.

Im not sure what else you want to hear or who you want to hear it from since im not a credible source. Im just stating facts.

 

You're right, Sideways suggested a solution, and the rest of you made assumptions and smart comments.

 

I'm pretty sure you can use the 3.9 mazda ring and pinion from the 3rd gen automatic RX7, should be a relatively inexpensive part.

someone please correct me if I'm wrong.

 

pretty sure i have you a few solutions too there buddy, you're welcome, lol