If you want to build a miata in the name of fuel economy, your first modifications would probably not be in the area of the drivetrain.

I am not building "FOR" economy, just trying to keep it in mind while making my build decisions.

But I do wonder what a high compression build could yield for economy? I know they are more efficient but if we are talking about <5% then it is a mute point. I am very happy with my build as is......but since everyone else wants taller gears I am looking into that. I have the tools, time, and desire to not be limited by what is a drop in replacement. Just wanting to learn and make an educated decision.

Thank you for your inputs!

See that's the thing, I'm not saying you can't or won't do it. I'm saying if you wanna do it in the name of fuel economy it will be a giant waste of time and money.

But for power/reliability reasons its actually a good idea.

If I was in your situation and decided to do the ford or chevy or whatever swap, I'd go 3.3 or lower. Your EFR should provide enough lowend/midrange for the car not to feel sluggish

*edit: and lastly, IMO you won't get any better than about 10% fuel efficiency increase no matter what rear end you end up with. We are working with 80's technology BP motors after all. Just my opinion

I would be very happy in this range. I know there is a lot more to be had with my tune as well. I will really start concentrating on that once I get the hardware I want in place. So then the question is should I buy the 3.6 or just live with the 3.9 that is already in the car?

difference between 3.9 and 3.6 is really not that significant.
also if you're still running the tune you showed me, there is definitely a bunch of room for improvement. its ultra safe...but that's probably it

Yes, same tune for now. Will try to get my D585 coils and intake manifold on soon so I can get back to dyno.

Have you seen my intake manifold thread Vlad? When I get back from a work trip next week I plan to get working on that. I see a Flat top vs Honda vs custom intake shootout in my near future. To maintain daily drive ability I decided to go with the 64mm Skunks Miata TB.

A few years ago I towed a small 6' enclosed trailer with my MSM and 3.63 gears. I got shit mileage with the drag of the trailer. ~18mpg at around 65mph highway in 6th gear. On one of my road trips, I noticed the boost gauge was up nearly static cruising like that and the ECU was tipping in fuel accordingly. I shifted down to 5th, held the same speed and dropped to ~10in/hg of vacuum. A full tank later I was getting ~24mpg towing at the same speed.

The drag of the trailer amplifies the phenomenon but it proves how turning lower RPMs doesn't necessarily save fuel. Load and the engine's torque curve has a lot to do with it.

Do the 3.63 gears but not to save fuel. Do it so your nice street car won't feel like a buzz box on the highway. :party:

With the 3:636 it wouldn't surprise me if peak fuel efficiency was when cruising at about 75mph to 80mph, better have a radar detector. The Miata engine gets shitty fuel economy operating that far down the torque curve. Id bet the 3.909 would do better.