custom transmission 6th / cruising gear?

I've been researching transmission swaps involving the Nissan CD009 transmission.  The 6th gear is 0.794 and I'd like something numerically lower (in the 0.6x range) for a lower cruising RPM on a car with smaller diameter wheels than the Z and limited final drive options (3.9). 

How feasible is it to get a custom gear done?  Would the transmission itself also require modifications to work with the lower gear?

Does anyone know of a company that would make a small batch?

Thanks. 

Anything is feasible if you have enough money.  With a design and everything ready you'd probably be looking at a thousand dollars for a one-off machining job.  You'd likely need to heat treat as well.

I'd try and find a gear that fit in there already, maybe from the same transmission family or something from a different position in the same trans

The required mod would need at least two new gears to be made.  The driven gear would need to loose teeth and the drive gear to gain teeth.  If there's room in the case for the larger drive gear it may be possible but that's a big if?   Of course the design of the trans will determine this as well.  Is the trans a "2 shaft" trans or a "3 shaft" trans.  I would bet a conventional 2 shaft as most RWD trans are.

As to who can do this,  I'll have to punt on that question.

Isnt the CD009 just a Aisin AZ6? Any chance you can swap a gear from a different AZ6?

Edit: Photo evidence doesnt add up to the AZ6 and CD009 being the same. Might both be Aisin though...

Might be more cost effective to swap in a differential that has better ratios availble than making a one-off gearset.

I think it would be cheaper to buy a different car with the proper ratios.  Custom gears are very, very expensive.  As said above, I'd work on the diff ratio.  You have six gears, there's gotta be enough room between first and sixth to get what you want somehow.

But then, powertrain engineers have never really impressed me with their "Real World" applications.  Sixth is either about 5% off fifth, or the engine is turning so slowly you can't shift it into 6 below 96 mph.

In reply to Streetwiseguy :

I am assuming the trans is going into an RX-7 as 3.90 is the tallest rear offered in a Mazda 7" and the tires are wee.  That or a Miata which works out to pretty much the same 22-23" tires and Mazda 7".

In either case, personally I would not want a gear any taller than something in the .8 range, like Mazda did.  I've used FC transmissions with the .697 5th and the 4-5 drop was unbearable.  As you point out, you find yourself never really able to use 5th unless the road is flat or downhill, or you are going fast enough that the revs are up over 3000.  Which is haulin' with .697.

It was nice with the 4.78 gearing though, except for how easy it was to destroy the 5th gear synchro becauss of the huge rev drop.

^If that is the case, you may be better off looking at an aftermarket ring and pinion http://www.miataroadster.com/mazda/mazda_ring_pinion_sets/g-63127.aspx

That said, if a CD009 is needed for the swap presumably it has a fairly powerful engine in front of it which could break a Miata R&P so maybe a diff swap would be productive.

In reply to ProDarwin :

CD009s are turning into popular swaps into RX-7s too because apparently unlike anything Mazda put behind a rotary, the synchros actually tolerate shifting at 9000+ rpm.  Normally you'd either get a dogbox or good at transmission R&R, or just be patient with the shifting/save the high RPM for entry into braking zones where you were not going to upshift anyway.

6th gear in the FiST is o.690.

Not that it helps here..