A number of members (but probably a minority) have concerns about the ratios of the 5-sp auto.
In particular the very low 1st & the fact that o/d is, overall, lower than in the 4-sp.

I'm curious to know whether its feasible to make any changes to the gearing.

At 100 kmh the 5-sp cruises at about 2300 rpm (depending on tyre size).
Changing to the 4-sp final drive would bring that down to 2050.
Going a step further & also substituting the o/d from the 380 would give 1925 rpm.

If you wanted to make the overall gearing in 1st the same as in the 4-sp (by changing the FD only) you would need to use a 2.45:1 FD. That would give 100kmh at 1540 rpm!

The other way to go would be to fit a 380 auto (1960 rpm) - has anyone looked into how difficult it would be to install & how confused the electronics would be?

Think lots of $$$$$$$$$$$$$$$$$, easier just to change the rolling diameter of the wheels (new tyres or change wheels).

I think overall if the aim is to save on fuel costs it will never be worth it.
Because, using 300rpm less, but more load, would not save a huge amount of fuel, and it would take too long to make up for the cost in simply fuel savings.
If you want economy, LPG conversion may be feasible depending on how many km you drive

I'm sure the Mitsu engineers have found the sweet spot between available power at cruising speed and good fuel economy.

If you are just after the lowest rpm at cruising speed, buy the smallest car you can find with the largest Diesel engine, and specify the manual gearbox option.

380 auto doesnt fit, and why the hell would you want to use the first gear from a 4sp auto?! It's HORRIBLE! It's like taking off in second in the 5sp... To answer your question it should be possible (matter of changing pinion and crown wheel) but you would need to be prepared to spend the cash, and do your research first to ensure the pinion and crown wheel will fit

No it is not a conventional diff with a crown wheel and pinion.
More like a drive gear that is part of the transmission output shaft and the driven gear that looks like a crownwheel.

Unless there is a JDM or USA transmission that has a lower numerical drive ratio then the only way will be to have custom parts made.

Not cheap.
Gerry

No it is not a conventional diff with a crown wheel and pinion.
More like a drive gear that is part of the transmission output shaft and the driven gear that looks like a crownwheel.

Unless there is a JDM or USA transmission that has a lower numerical drive ratio then the only way will be to have custom parts made.

Not cheap.
Gerry

Bad terminology...

There is a servicable DRIVE gear on the output shaft and a crown wheel bolted to the diff centre. The 5sp uses different driveshafts so even if the crown wheel fits you'll need to use the 5sp diff centre.

380 auto doesnt fit, and why the hell would you want to use the first gear from a 4sp auto?! It's HORRIBLE! It's like taking off in second in the 5sp...

Interesting, I find first in the 4spd box to be very zippy.

Interesting, I find first in the 4spd box to be very zippy.

Have you driven on either of the 5sps? Auto or manual... Take off in second in either and thats first in the 4sp.

Have you driven on either of the 5sps? Auto or manual... Take off in second in either and thats first in the 4sp.

I used to have a 380 (auto). I don't first in the 4speed to be sluggish whatsoever.

To address a couple of the comments:

I was assuming that any changes would only be made if a 5-sp had to come out for repairs. (This would partially address the cost issue.)

My car is 4sp - I only asked the question out of curiosity. My concern with the 5-sp is not so much about fuel cost as with the slightly less relaxed cruising - despite having an extra gear. It just seems a backward step, especially when compared with 6th in a Falcon, Commodore or Aurion.

I don't know about 1st in the 4-sp being "zippy" but I like how I can get a decent burst out of it before it changes up at still low revs.
Please remember this is from the perspective of a Verada wagon owner, not a VRX owner.

Possibly there should have been 2 FD ratios - a higher one for later (5-sp) Veradas & maybe the VR & base AWD. Then a lower ratio for the VRX & VRX AWD (& Verada GTV-i?).

I've had a look into this before. But not too seriously. The F5A51 (five speed) trans is used in other vehicles as well. Some rudimentary searching reveals there were F5A51's with different ratios. I'd need an expert to tell me that the internal "mechanical" workings varied too such a degree you could switch parts between them. I can understand the electrical/hydraulic relationship changes between them. I just cant see there being great mechanical variance between them as the cost to revise the transmission would be expensive. Design/build/test/mass production of an auto trans would have to be astronomical as it is. Changing the the final drive ratio would be easier for a manufacturer... easier than altering the gear ratios (planetary gears, interrelated ratios bla bla).

I going to be daring and say this would be plausible - MECHANICALLY. And only expensive to do if you didn't research it properly. There would be some F5A51 out there with a 3.333:1 final drive that could just drop straight in... but which car has it? Which model of the F5A51 is the closest to the Magna five speed? Find that model, and which car it was standard in, then off to the wreckers... it would be nice if it was that simple.

Another drawback is that the overall ratio is linked to the speedometer, so the speedo drive gear needs to change. And there's an output shaft sensor on these transmissions - what happens when it gets a different signal to what its expecting??

I'd be asking around with automatic transmission experts... somebody who has been in the game a while and has access to information not available to the general public. More research will offset the costs in varying degrees.

Another drawback is that the overall ratio is linked to the speedometer, so the speedo drive gear needs to change. And there's an output shaft sensor on these transmissions - what happens when it gets a different signal to what its expecting??.

Not an issue with the speedo, the magna runs the same sensor on 4 different transmissions (4sp auto, 5sp auto, 3.0 manual, 3.5 manual).

The sensor counts - the ratio changes. Input shaft sensor says (counts) one thing and the output shaft says (counts) another = confusion for the TCU.

Scratch that. Output shaft wouldn't be affected by changes in the diff ratio.

Just having a look at the sectional view of the five speed auto - it looks like the drive gear for the differential is splined to the shaft and the driven gear is bolted. So... if you knew somebody who could cut gears...

Working off a line drawing in a manual = questionable.