The elusive 3.308:1 rear diff
 

Ok, so. First I realize that ALL of the 3.308:1 rear gears in America for a mazda are the 7.5" ring gear and NOT the 7" ring gear that we use. It is available in the manual tranny mazda b2000 pickup truck of the 82-84 vintage. I've found quite a few tiny tid bits of what I'm confident is good information. So, I know that the miata diff cover piece will bolt to the b2000 pumpkin, they have the same bolt pattern, grinding may be required to clear the bigger ring gear. I also know that the 2wd truck uses the same diff carrier as the 84-88 rx7 that we can swap into our diff, so in theory the carrier would be the same as the miata's. You need to swap the carriers because the splines for the axles in the carrier for the truck is bigger than the miata axles.

So my parts list of what I believe you need
2wd manual tranny 82-84 b2000 front diff section
miata diff cover/mount bit
any miata diff carrier (torsen, open, giken)
new bearings might be a good idea

So procedure wise
take stock truck diff and miata cover, line both up and keep grinding stuff off the miata cover until there is no more interference
pull the carrier and ring gear out of the diff housing
pull the ring gear off the b2000 carrier, bolt it to the miata carrier
put that into the b2000 housing, set preload and backlash
put miata cover on, put back in car, fill with fluid, use the axles already in the car

I'm not 100% on the diff cover bolt pattern being the same. But b2000 diff assemblies are so stupidly cheap around here that honestly money would not stop me from trying this. Time is. Time is the reason I dont already have one of these setup. I'll have time this summer once the rest of the car is together. Anyone else wants to try this before then, be my guest. IMO it makes the 6 speed a viable option, and I think the bigger ring gear with a giken would make this rear end bullet proof for the amount of power than pretty much anyone on here makes.

What year B2000? I think it has to be 82-84, no?

79 80 81 82 83 84 Mazda B 2000 Carrier Assembly | eBay

Food for thought
"MAZDA B-2000 79-81 (3.31 ratio)"
"MAZDA B-2000 82-84 MT (3.31 ratio)"

Yes, I had to do some more searching on that one because some reports were stretching that all the way from 77 to like 88. And car-part thinks that as well, heck be careful with car part because if you put in the 84 b2000 axle assembly and choose the 3.31 ratio it will throw in the random 70's toyota land cruiser too.

1 more hitch that I have not seen mentioned is that I am not certain that the distance from the input shaft flange to the axle centerline is the same. And that would require a custom length axle if it was different which would suck a lot.

You're forgetting about the PPF me thinks.

Completely over rated, we have a driveshaft to keep everything aligned.

Aligned-ish is good enough.

Shit. Yup, no PPF mounting. Well that cocks up that idea. Now it basically has no advantages over the putting in a ford irs.

But it does use the samish length pinion shaft as the miata. I say samish because it keeps getting refereed to as "the short one like the miata so it wont work in you FC"

That will give you an opportunity to hone your fabrication skills. Don't give up now.

What, welding PPF mounts onto the b2000 diff housing? F that, I dont want to risk warping it.

Front diff.

There are a million ways to skin a cat. The same could be said for mounting a diff. Motivation to me would be a stronger, lower ratio than the miata 7", but not the power robbing overkill of the Ford 8.8".

I'm failing to see why you are worried about warping it :giggle:

Interesting academic thought experiment though, assuming you don't try it of course.

Yeah, I'll +1 and "cool story bro", and wait for someone to actually do it rather than hypothesize

There is only 1, its 117 miles away from me, and I dont know if it has the correct ratio. Though I had seen mention in a couple spots that they are in fact the 7" ring and not the 7.5".

And... there is a 1982 manual diesel b2200 20 miles from my house...

Have em measure the diameter and count the teeth.

I have. Called the close place, they scrapped theirs, the place in NY is snowed in but will check when they get the chance. I also called mazdaspeed, and here is where it gets interesting. The part number for the 82-83 diesel manual 3.31 R&P is the same as the 82-84 B2000 auto trans 3.31 R&P. BUT his documentation shows that both of them are the big gear, it does show though that the 82-83 diesel R&P is a different part number than the 84 diesel R&P. He said though that they have 3.63 sets for the 7" ring coming out their ears, that I already knew.

I always considered getting a six speed and getting the shortest final drive available to make third the ideal gear for autocross. 4.778 or the 4.625 from a Sportage. This makes the sixth gear highway cruising pretty awful considering the 6th gear ratio is already shorter than the 5th gear in the 5 speed. I did some part number research though and the main and counter 6th for the Miata 6 speed matches that of the early RX8. The later RX8 got a taller ratio for sixth gear so that could be an option to alleviate the very high cruise rpm with a 4.778 or 4.625. That's what I would do to make the six speed work for autocross and the street.

I made this little table that demonstrates what I'm talking about. You see that if you swap in the newer RX8 6th and run a 4.625 you have a third gear that is virtually identical to the 2nd in a stock NA and your highway cruise is less than 200 rpm more than it is usually in a stock NB. Plus you have shorter gears all around which makes the car more fun anyways vs turning the car into a total dog with an ultra tall final to get the 2nd gear to work.

Max speed autox

5 speed 2nd 4.1 64 mph
5 speed 2nd 4.3 61 mph
6 speed 3rd 4.778 63 mph
6 speed 3rd 4.625 65 mph

80mph top gear

5 speed 5th 4.1 3890 rpm
5 speed 5th 4.3 4080 rpm
6 speed .843 6th 4.625 4550 rpm
6 speed .843 6th 4.778 4700 rpm
6 speed .787 6th 4.625 4550 rpm
6 speed .787 6th 4.778 4250 rpm

You can see here how the 6th interchanges. I would be pretty surprised if the revised 09+ .787 6th didn't also interchange, maybe someone has tried it?

GEAR, 6TH. 2004 Mazda # Y61117631
GEAR, COUNTER (6TH). 2000 Mazda Miata # Y60117309