Diff swap on a Toyota Celica, for economy at high speed.
 

Anyone know about any diffs that would fit on my 1986 Celica? It's current final drive ration is 3.944:1... looking for something closer to 3?

I doubt you can swap your differential out for one that is geared differently. The pinion gear on the transmissions secondary shaft is likely cut into the shaft, not bolted on. Therefore, to change the differential gear ratio, you need to also replace the secondary transmission shaft which is not a small job. If it is bolted on you're in luck.

Fair enough. Perhaps a specialist job then?

How hard would it be to swap out 5th for something a bit smaller?
I'm pretty sure my 5th is .82? I sit at 100 on the highway and it's ringing up at 3,000 nearly... :/ can't be good for mileage.

Sorry, I was wrong! :) I just looked it up. The differential swap is possible. It is not cut into the shaft (5th gear is however making that a custom job).

Good write up on a Toyota trans rebuild:
C52 transmission rebuild

Sadly, I don't think you'll gain a ton of ground there. The lowest final drive I know of is a 3.722 for a C series transmission which I am assuming you have (not totally sure). That will only drop your rpms by about 160 @ 100 kph. The only thing lower is what they've been putting in the diesel transmissions. I don't know what is available for you in Australia, but if you could find a C series trans out of diesel you'd be much better off just swapping that whole thing onto your bellhousing.

I think my transmission is an s53, or s52, but yeah it seems all people want to do is drop skids, ahaha loads of info if you want burned rubber, but not much for fuel economy lol
any ideas on that? :)

Sorry, I'm really not familiar with the S series of transmissions. I'd just try to look up gear ratios of cars that use S series transmissions and see what you can find. That is how I gathered all my C series info.

Thanks mate, I'll let you know how I get on. ;)

I know for the C series transmissions there are also 6 speed options. (C160 and C60) Both are good options for fuel economy. If I recall correctly the S series is the same as the 90-95 MR2 transmissions. The only 6 speed that I know of for that block is for the Beams IS200 rear wheel drive application. I think the only major difference in the S transmissions were the side for the shift linkage (MR2) and the LSD option.

Unfortunately, the 6 speed C series has very short differential gearing (C60 is 4.529:1, C62 is 4.312:1). So, even though you get an extra gear, it doesn't help in on the top end. All the 6th gear ratios I have say they use a .725:1. Almost all the 5 speeds use a .815:1, although some use the .725:1.