Gearboxes?

[Swing]

I'm looking for the best gearbox with the best engine for having the best fuel usage in a mk1 Yaris (P1). Currently I'm getting over 5L/100km so up to 50MPG with my Yaris 1.0, but I cannot take it any further because this little engine is working hard on the highway, with high rpm. Earlier I owned the 1.3 version, and I got similar results quite easily, so with my slow driving, I don't need this really small engine.

So there has to be an ultimate combination of an engine with a certain gearbox to get the best fuel usage.

Wikipedia has a nice list of gearboxes suitable for the Yaris platform, the C transmissions: http://en.wikipedia.org/wiki/Toyota_C_transmission
But still a lot is unclear, for example, final gears. But for the petrol versions of the Yaris, that is probably around 4.0 for the C54 gearbox (used on 1.0 and 1.3 litre engines it seams).

I'm looking at the diesel gearbox C53, which has very long 5th gear. Not sure what the final gear is. Questions are:
- Can they be interchanged? Probably only the housing towards the engine is different because of different engine block? Driveshafts etc are the same?
- Or can you open a gearbox and use the 5th gear(s) of one gearbox inside the other, since the form factor is probably the same.. ?

I also sometimes read about changing out final drives. Is that supposed to be easy?
Another idea could be, take the 1.5 (TS or TSport) Yaris with 1NZ-FE engine (still FE, still Fuel econmy :P ) and change the gearbox to simply have a longer final drive.
Gearbox of the 1NZ-FE engine seams to be C50, which already has a longer 5th gear and a final drive of 3.722. That means it already has a pretty good rpm when you drive 100km/h, so with my current slow driving style, I probably get the same results on a 1.5 Yaris.
If I would put a longer 5th gear or a longer final drive in there, it could do very nice, coasting around on low rpm on the highway (but not too low). For the engine it should be pretty easy, getting this light car to coast around 100km/h or slower.

But if it has a final drive of 3.722, there are probably not many options for final drives. I think the longest final drive in C transmissions is 3.550, which is the C551 gearbox for newer Yaris models, or for Toyota Aygo I think.
But that's only a small difference, you would like a bigger difference to make it worthwhile. For that small difference I'm not going to open the gearbox.

Any ideas on this?
(I know the diesel Yaris is the most economic one, but road tax is way to high, and nice diesel versions are hard to find for the mk1 Yaris)

[CrankyOldMan]

I've got a thread showing the difference between a few C5x gearboxes, The biggest difference is the bell housing (the casing part that goes on the engine) and the location/presence of mounts.

From what I've seen , the C5x parts that I have are all interchangeable. I haven't looked at the C52, 53, 54, or 551 as those aren't readily available over here, but there are a LOT of identical internal components. The 5th gear set seems to be interchangeable, so I'm going to use the 0.725:1 set from an 07 Matrix (Corolla wagon) instead of the stock 0.815 on my custom gearbox.

Spend some time searching/browsing the forums, there's lots of info out there.

[why?]

For Yaris gas mileage nothing beats the D4D.

should have read teh entire post. But you already have teh best engine. I'm getting about 6l/100km with the 1.5, there are a few that get better, but not very many. My best was exactly 5l/100 km.

here is the converter I used.