I had asked the same question back in 2012, not that it matters.
lotus-gearbox-f37/4th-gear-adding-overdrive-driveshaft-t25659.html#p167776There are many gearbox threads. MX5 6 speed etc. I would prefer a Nissan CD009 over any. But that?s me, unfortunately it is almost twice as heavy at 148 lbs.
And there have been many who are happy with their 5 speed. As I read it, a 5 speed without changing rear diff only increases top cruising speed minimally.
Of course a 6 speed mx5 would be the choice (as you can only get an 8 speed in a auto at this time). If I or someone had taken the steps to make adaptor plates. But there is a reason SJS (i could be wrong again, might have been TTR or Sue) put a 6 speed mx5 box in an elan with the intentions of going into production.
So I waited, as I dont live near a city, and do no. And dont have time or money to make a plate myself. Though i suspect there are other hurdles. Might have been angles of shaft, or spline or something. Again, that is not the only 6 speed box, but the mx5 6 is the lightest i believe.
Many people have their opinions on why the original close ratio box is best. I tend to side with them. Not saying I would not go with a 6, 7 or 8 speed.
I believe the reason for a Over/Under drive is cost, fitment, weight distribution (if one considers diff mounting). Of course I could be completely wrong on all of those points.
A used MX5 6 speed is about $600. One would consider a rebuild as the ones listed on car-part.com (no affiliation) have high mileage. So that gets the costs to about what a GV unit should be. Then another $400 for adaptor plate as long as that is it. Close to the same costs.
Six speed transmission from MX5 including clutch fork, but not throwout bearing or shift lever: 87.5 lbs
Maybe someone knows the weight of the 4 speed. I would guess they are about the same, and someone might know the length of a mx5 box. Which I would suspect is longer. So possibly adding more weight to the front. Which is probably the opposite of what the baby elan needs if memory serves, it needs another 50lbs in the rear. Again, i could be very wrong. Merely asking.
Yes, a 5 speed would be quite capable of cruising at 120mph. Not sure about tyre size and gear ratio, at what rpm... I bet someone knows. So if the costs for a proper built 5 speed is $3500, and new diff ratio at $700 it is a bit more costly. And arguably lower top gear. I could/should/might have gone through the ratio calculator and figured it all out.
So I read it like this. 5 speed $4500ish vs 8 speed $3000ish. Do correct me if my math is off. I suspect a rear diff mounted over/under balances the car. Further if there is good reason the 4 speed is rubbish. I enjoy both of mine! And ultimately, dont want to change, but I do, and have for a long time wanted overdrive units. Might be because my Volvo estates have electric 5th. Though I have a CD009 adaptor plate for one of mine.