My car runs about 4000 rpm at cruise and there's a partout near by with a 3.9 Torsen and 6 speed, I have the 4.1 Torsen 5 speed. I've read that the 5th gear in my trans is all but identical to the 6th in the other and was wondering if the 3.9/6 got me a lower cruise rpm without a sacrifice in acceleration. Did I see somewhere that the 5 is stronger? Shift quality better in one vs the other?
Generally a slow car that you have to shift more often will feel less slow. I dont know if it's worth the effort in objective terms but if it's cheapish and you want it i doubt you would regret it.
In reply to AnthonyGS :
That's in line with what I've read
Generally the 5-speed shifts better and the 6-speed is stronger. The 6/3.9 will have lower gearing everywhere, meaning that it's generally faster unless you're measuring to an arbitrary end speed that happens to be 1 fewer shift on the 5-speed than on the 6. (the fact that some cars do 60 mph in 2nd and others do it in 3rd is one reason why this metric is much less useful than a distance-based measure like 1/4 mile times).
The 6-speed will not give you measurably lower cruise RPMS unless you pair it with a 3.636:1 rear end. It's a close ratio transmission, not a higher overdrive.
Who cares which is faster to 60? Regardless, it's pretty well known that NA, a 6 speed car (even better with a 4.1) will be faster on almost all roadcourses.
A 5 speed with a 4.1 is my favorite transmission behind a 1.8. I just like the spacing of the gears, I think it's well suited to the engine and of course the shift quality is better. The best 6-speeds are almost as good as an average 5-speed in that regard.
6 speed 4.1 (aka 2004-05 factory spec) is annoying on tight courses, which is why we swapped our shop MSM over to a 5-speed. Heretical.
If you want to play with numbers, https://www.flyinmiata.com/tech/gearing.php.
It's pretty clear that a 5-speed 4.1 will take one less shift to get to 60 than a 6-speed 4.1. Even a 6-speed 3.6 is going to just barely able to hit 60 in 2nd (assumptions: 7300 rpm redline, 205/50-15 tires).
I think the question is not so much which is quicker, as which is less slow.
3.63 is what I want in the pumpkin of my Miata
When i was working on that turbo NB i posted about recently in my 3.63 gears thread, the one thing i really wanted to do to that car (if it were mine) would be to swap it to a 5spd. Even with 3.63s the gear spacing was narrow considering the power level it was at (in my opinion). I do think that where you prefer your shifts at changes depending on how fast a car goes through the gears. When i had my first manual trans Minivan it had a WOT shift every 25mph and that made it feel a lot less slow than it was. It's pretty universal that more gears makes a slow car more fun, and when car start to become very powerful they take gears out of them. Most of the fastest manual trans drag cars are down to 4 gears, the fastest auto trans drag cars have 2, and you can buy a Koenigsegg Regera with 1 gear.
When you put a timer to it, i think the 5>6spd swap on an otherwise stockish car won't pay much dividends. If you're doing it for fun, by all means do it.
How it is now, which is basically stock, my only issue is the fairly high rpm at 75 mph. If the 6 speed/3.9/3.6 yielded the same or better acceleration AND lower rpm at 75 it may be worth it. BUT, after reading yalls comments it sounds like I should leave it until I make more power (I can has turblow?) then swap in a 3.9 or 3.6.
In reply to Justjim75 :
That's one thing i'd like to address, too- and if you read Doc Brown's 1.3l build, he shows a combo 5th gear and rear end change. I may try to find a spare 5 speed and a lower rear end and try that change.
The other possibility is and oddball RX7 box wth a 3.6 final- which puts 1-3 almost the same as stock 4th taller, and 5th MUCH taller- like 25%. But the page I found all that info on went away a long time ago, so I can't recall what gearbox that is, or even if you can use it in a Miata.
The other possibility is and oddball RX7 box wth a 3.6 final- which puts 1-3 almost the same as stock 4th taller, and 5th MUCH taller- like 25%.
That is VERY enticing, assuming it's not ridiculously hard to find the donor.
How it is now, which is basically stock, my only issue is the fairly high rpm at 75 mph. If the 6 speed/3.9/3.6 yielded the same or better acceleration AND lower rpm at 75 it may be worth it. BUT, after reading yalls comments it sounds like I should leave it until I make more power (I can has turblow?) then swap in a 3.9 or 3.6.
Going all the way to 3.6 is definitely going to cost you a chunk of power. Whether that matters depends on how much you care about the acceleration you have, and how much you care about that highway rpm. For me the highway rpm factor is huge and i sold the only Miata i owned in part because of it. I drive cars slower than a stock Miata all the time and in general i feel like adding power is often easier than lowering highway rpm, so I'm more likely to take a car i like on the highway and add power than take a powerful car and lower the highway rpm. I did make a thread recently about the 3.63s in a turbo car, you might find it interesting. I would say it's a bigger vote for turbo than it is for 3.63s, but if you do both im pretty damn sure you'll be happy. 
In reply to Vigo :
I'm with you on this. From an offroaders experience, there are 3 ways to improve power on the rocks, gearing, horsepower/torque, or weight reduction. Sounds like I may need a little of the first 2 in the long run, dont see much room for any significant weight reduction tho. I mean, air conditioning in Alabama is second only to fried pork chops in terms of necessities
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