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has anyone ever swap the k8 final drive with another final drive from other gearbox?

The complicated answer is yes
The simple answer is no

The final drive is determined by the ratio between the pinion gear and the differential. The differential is possible to change, but the pinion gear isvery difficult if not almost impossible since you have to remove it from the secondary/output shaft, which is a pain for what I've been told.

Not many have tried taller gears on a K8 mostly because of the lack of power, it's suspected that although you will bring the revs down, the motor would have less leverage, so it would actually require it to work harder. The question is if the lower revs are indeed beneficial and would offset the extra power required to turn the motor at those RPM's compared to the stock gears. I know that most people are concerned with their milleage considering that you're reving around 4000rpms doing 120km/h, but you also have to consider to how much you're pressing down that throttle. I've been 5 years since I've had a K8, but I know with my ZE, doing those speeds with the K8 tranny I''m not pressing it down much.

You have a few options. One thing you can do is swap to a taller 5th gear for cruising at highway speeds. The Mx-6/626 have a taller 5th gear and that's swappable with the tranny still in the car. The other option is actually swapping to an Mx-6/626 tranny, that has a taller final drive of 4.11:1 compared to the Mx-3/Probe's 4.38:1. Of course the Mx-6/626 has the taller 5th gear, and because of it's taller final drive, it lenghtenes 5th gear even more compared if it was paired with a 4.38:1 tranny.

There are other 5th gear and gearset swaps possible, if you search around this section,you'll find some info on them. They are pretty much even taller ratios however, so if you're sticking with a K8 motor, i''d say don't try any longer than what's mentioned above. Like I said, no one has tried it, so there may be no benefit or even a draw back to running that, but it's all just speculation until someone tries it.

Actually if you look at Mazda's OE specs over the years you will find that Models such as the (BG) Protege had a F5MR with 3.62 final drive ratio behind the 108hp B8-ME. I used on of these with a (8V) B6 SOHC for a few years with less wear and tear on the engine due to lower engine speeds. Also 3.85 gear sets were used in models in Europe that have the Z5-DE and BP-DE. which have similar power rating as the K8. Transmissions are rated by torque handling capacity not by engine power. Manufactures usually pick gear ratio based on such parameters as vehicle mass, intended driving conditions, the peak torque of the engine and the rpm peak torque is achieved. Lower gearing results in more torque multiplication. This is the primary reason for blowing trannys when swapping to a bigger engine or adding FI you have increased the base torque going into the tranny that once it goes through the gear the torque is multiplied beyond the mechanical limits of the gearbox design. Mazda in general tended to use close ratio in their gearboxs in their cars for Japanese driving conditions lower speed, climbing inclines, multiple started and stops and much smaller engines rather than for high speed cruising.

Up until a few week ago when I sold the car, I was driving with a built up G15M-R with mixed and matched parts with a 3.41 final drive. and 0.68 5th behind a slightly built up '98 2.0L Zetec. This MTX featured wider steps between gears which used the power the engine developed more efficiently and put less stress on the internal componants of the engine and tranny even thought I used the beefier OE parts from '03 and later gearboxes. It was a little slower off the line but part of it was made up in the fact the it would go off the line without much wheel spin and I could stay in gear longer between shifts. Plus the gas mileage was well below 6L/ 100km for mixed city/ highway driving a big bonus when gas prices were at there highest earlier this year.

has anyone ever done a write up on the 5th gear swap into a mx-3 trans? I just picked up a 96 mx-3 gs and I hate that its geared so low for highway cruising. 5th really needs to be taller on it. at 120km/h I am doing about 4000 rpm, way to high for highway cruising. or should I just swap in a mx-6 5 spd trans when I do my klze swap.

nerd_racing did one. its in the FAQ.

Newfie_dan wrote:has anyone ever done a write up on the 5th gear swap into a mx-3 trans? I just picked up a 96 mx-3 gs and I hate that its geared so low for highway cruising. 5th really needs to be taller on it. at 120km/h I am doing about 4000 rpm, way to high for highway cruising. or should I just swap in a mx-6 5 spd trans when I do my klze swap.

I did one on the yahoogroups MX-3 list back in about 1998 or so when I first swapped the 5th gear set way back then. With the KL you should really swap to the 4:11 to match the gearing to the engine out put and best as possible without going through the trouble of building a Frankenstein tranny.

Would you post a link, or some idea how to find it?

This topic on iMazda.com shows a very thorough G series teardown and rebuild with LSD and 5th gear swap. It is the best write up I've seen and will tell you pretty much everything you have to know about how to do the actual build. As far as what are the correct parts and which transmissions have which gear ratios and so on, there are a few links referred to in that thread and there are others on here that offer good information.

I have swapped to a 4.11 tranny from a MX6 for my ZE swap. It's geared better in general for the more powerful motor.

It's the simplest solution. Instead of 4000 RPM you will be going @ 3500. Not a big difference... but over long distance it will save fuel.

What did the .68 5th gear come from? i have been searching around like crazy and cant find any info. I did find once b4

It was frankesteined together from a couple Euro import G series trannies. That's all I can remember.

oh ok, i have found out US aspires and festivas have .69 5th gears