Is the gearing on the FC ridiculous?
 

When I asked him about ways to make my NA not feel so damn sluggish, Kahren pointed out that the final drive in FCs doesn't really match up to the car very well. Sure, the 4.10 rear diff may be good for the turbo FCs, but it seems like it really kills the NA FCs. I've noticed the more I drive my N/A the more I notice this issue.

Gearing is not as big of an issue with S4s but it really shows on S5s. If I rev my S5 NA out to 8k in second gear I hit almost 75mph. Am I the only one who thinks that's a bit tall? Sure, 5th gear may be a nice overdrive, but it's so tall that the gear-limited top speed on S5 NAs is over 220mph! I've also found that at 2900rpm around 60mph, it's really impossible to accelerate much, if at all in 5th gear.

I know everybody says the GTUs 4.3 rear diff is a good upgrade, but I've been recommended a 4.8 rear differential by a few people now. This car is what I drive between school and home every two weeks, so do you think such a drastic change in the final drive would affect gas mileage much? I've run the numbers through a few speed/rpm calculators and it seems like it would make acceleration much better while keeping the car down to about 3500rpm@75mph, which I never really go over on the freeway.

Have many people done this upgrade? I just want more pep, a little bit more accessible power if I'm going through the mountains, or a little more oomph to pull me out of a corner, is this going too far?

I can only say this according to other 7 experts who have told me.

changing from 4.10 to 4.30 will decrease gas milage as you're now running at a higher rpm.

going to a 4.8 diff will help accel, but you're top speed will be like 8 mph (figuratively speaking). In a torque-less car like an NA Rx-7, accel. from 5th is impossible. I even think doing it in my old '01 Mustang is impossible. That's why when I had my 7, I'd rev-match and drop to 3rd.

If you never hit the freeway and long for better accel. and don't care about gas, do it. If otherwise, stick with 4.10's.

a 4.8 in a NA would be awesome! yeah, it would hurt gas milage. but not a shit ton... i drive about 80mph most the time. Thats right at 3k for me. I havnt noticed much difference in gas milage from 3k to 4k rpm. maybe a few mpg...

although, I know im not alone in this, but, my NA pulls just fine in 5th from 60mph all the way up to 130. I RARELY have to downshift on large hills to keep my speed from dropping.

in the books it says that n/a rx7's are geared to go 191mph but of course dont have the power stock but if you did the gearing could take you there
turbo's are not geared that tall though i think 17?mph

220 mile/hour (mph) = 354.055 68 kilometer/hour


dude you're dreaming if you think they're geared that fast


Max speed with the limiter and gearing is about 260-270 km/h


that sounds much more likely

4.1 (Rear End) * 1.0 (4th Gear) = 4.1 (Duh)
5.75 (Rear End) * .711 (5th Gear) = 4.1...

The only problem I can see is having decreased transmission life by running in overdrive all the time.

As long as you can keep it below 3800RPM's at cruising speeds you'll be golden... :)

Eh.

My 1st-gen has 4.78's, 195/50-15 tires, and an .825 5th, so I cruise at about 5k instead of 500rpm-per-10mph like stock. Fuel economy has changed exactly none, still getting 25-27mpg,with occasional bouts of 30mpg when conditions are just right. (Top speed is still about the same, for a 1st-gen, only now it powers-out in 5th instead of 4th. I don't see an FC having any trouble given they have tall tires and a waaaaaay tall 5th gear)

Gearing really doesn't matter all that much for fuel economy, the engine will still need roughly the same amount of fuel for a given HP and the car will still need the same amount of HP for a given road load.

Like 13b4me noted, though, stock EFI will richen you right out of good economy past 3800rpm.

I personally wonder if swapping 1st-gen guts into an FC transmission would be worthwhile. The 2-3 gear spread isn't so hot, but then the 1-2 spread is awful for the FC, and they're all taller besides.

What's the easiest way to get a 4.8 rear? Do you buy the gear or something or do you have to buy an aftermarket rear end?

You need an aftermarket ring gear and pinion gear that replace the stock ring and pinion inside the differential.

That's what I was thinking. On the S5 NA the 5th gear ratio is .697, so highway cruising would be pretty easy-going, the RPMS would be pretty equivalent to a 1st gen, and those seem to get pretty good gas mileage.

I have noticed the 1-2 spread in the gears is HORRIBLE. I'm dead even with my friend's Porsche 951 until I shift into second and get absolutely DUMPED out of the powerband, it's really annoying. But with a 4.8 rear end I figure I'd only be out of the powerband for a little less time. Here's the numbers for if I shift at 8000rpm in each gear with 255/40/17 tires:

4.10 4.8
1st 42 36
2nd 73 62
3rd 106 91
4th 145 124
5th 211 181

I think these ratios are much more friendly to a lower-powered car, and with gas mileage somewhat unaffected (I'll have to see), it seems like a worthwile upgrade.

The cheapest route to go for getting a 4.8 would be just buying the ring and pinion, but I currently have a GTU open diff, so I'm going to send it to ISC racing and get a LSD rear end built with the 4.8 final drive instead.

do a search for "miata gears"

and see I want a 6 speed with a even higher final drive, something like a .60 6th gear...

Just imagine at 100 mph, being at 3000 and running up from there.

I love the tall second gear. Yeah it sucks for dead stop accel; but how ofter do you do that? It really shines when you in some tight turns and you can leaveit in second and rev up and down without shifting.

I switched to the GTUs rear with the 4.3, and it was the best mod I ever did, but it made first gear almost worthless; a 4.8 would kill first gear almost completely. Not having a useable first gear really hurts in parking lots; I'm sure you're not woried about parking lots much; but you will be when you're surging around the gas pumps.

Go 4.3 or leave it alone.

Samps, with the gearing I have now, playing in the Appalachian foothills with an arbitrary 5000rpm rev ceiling (noise...) has me running no lower than 3rd gear, and the 3-4-5 gears on the 12A-spec trans are decently close. But when cone-dodging, I'm hoping to not have to use first gear *as much*.

I haven't dodged any cones yet with the short gearing, due to perpetual transmission failures, but I'm hoping to get another "spare" this weekend. Downside: It's from a Series 5, which means a horrible 1-2 and a horrible 4-5, so on the courses I might end up having to 1-2 it, and in the hills the 4-5 will suck complete ass.

The RX-8 gearset is looking nicer and nicer. Keep the stock rearend, enjoy the low 1st gear and the close-set 2-6. RX-8 6th is like a 1st-gen's 5th, or abiout halfway between an FC's 4th and 5th.

I belive 2-3-4 are your driving gears. 1 is just to get rolling, and 5 is for the highway to and from your real driving. I like the tall 2 because in parking lots or urban streets with 25 mph zones, 2 can do what you need without shifting. I haven't been in too many afordable cars with a good passing high gear. That is not what the high gear is for. In an automatic does the high gear do the passing? NO! It downshifts to a lower gear.

If you think 4.1 is slow, try the 3.9 that came in the verts.

I recently swapped in a 12A tranny into my S5 and it made a noticable difference. The 5th gear acceleration is now much better. I can actually go up some pretty good hills now in 5th, rather than having to downshift. There is a drawback of increased noise when cruising at a little over 100km/h from the 3000rpm drone (=~60mph, speed limits = 90-110 on most highways). The gears are shorter in 1st-3rd and much shorter in 5th, 4th is the same. If I had a choice I'd have gotten a GSL-SE tranny, as it's got a 5th that's in between the FC and the 12A ratios to give better 5th accel but with less noise. I think that would be a good budget setup. I can't say I've noticed any appreciable decrease in milage, but there's probably some decrease, I haven't really been driving it with the new tranny for long enough to be able to tell fully. It should be better for autocrossing due to the lower first and second gears, as the autocrosses in my area tend to be pretty tight and slow compared to a lot of the American events (so I've heard). Sometimes I don't even get out of first gear.

The reason why I did it is that my original tranny had a leak in the front seal, so I had to take it out anyway, and I got the 12A tranny with an FC bellhousing and tailhousing for $200 CAN delivered.

I think it was definetely worth it, despite the droning at ~110 from the engine being at about 3000rpm.

I've got a T2 tranny and rear diff in my s4 N/A, and its alot better than stock. The main issue I have with the gearing is first gear. It sucks.. about 4k rpm, I just hit ~18mph... but 2, 3, 4, 5th are great though. I have never needed to downshift from 5th to climb hills. I can even accelerate uphill in 5th from about 50mph. Although from 65mph+ it really pulls in 5th for an N/A. I do have an issue with my exhaust resonating around 3.5k rpm going 75 on the freeway... its much to loud.

i'm working on scoring myself an rx8 box, mui bueno

funny I thought the fc gearing was pretty good I have a S4 na and compared to most 4 bangers it actually pulls in 5th on the highway. I think the 4.3 gearing would be perfect really.

i dunno i about NA's but it it feels just right on my t2

it makes a lot of sense that for a less torquey car, the tranny would feel inappropiate

look up miata tranmission conversions, its supposedly really easy to do, and hte miata is a very, well geared car (the NA rx-7 has nearly the same power and weight, so its a good match)

I would like to know where to get a 4.8 That would be great for a drag car.

You can get a 4.8 diff from ISC: http://www.iscracing.net/axle.htm

Expect to pay several thousand for it though.

thanks for the link.

to convert you 4.1 na rear to 4.88 form isc racing is less then 1k not several thousand.
gearing in the miata trans is LONGER then the FC EXCEPT for 5th gear. so every gear except 5th will end up redlining at a higher mph except 5th. the miata gears are closer together then the fc na gears thought and if you road race that can come in handy with a 4.88 rear. putting just the miata gears in the fc na box will just make matters worse, and if you are gonna argue that you drove a miata and the gears wind out very fast with the same 4.1 rear in the miata, then dont forget about the small height of the tires that the miata uses which has a LOT to do with what speed each gear goes to. all this can be calculated with some simple math.