Diff Ratios

If it is an E153 as used in the MR2 Turbo it's final drive ratio will be 4.285.
That is the lowest ratio available that I'm aware of.
Honda transmission ??

OK. In that case it's final drive is most likely 3.944 to 1.
Where are you located? Which transmission do you actually have?
E153 or S54 ?

This is Thomas Cran's old car, it's an F20C motor fitted to an S54 gearbox via an adaptor plate. 
Build thread here: https://www.mr2australia...mr2play/tm.aspx?m=96882

I've done some quick calcs on an online gear/tyre/rpm calculator and come up with the following results, assuming your rev limiter is 9000 and rolling tyre diameter is 620mm (235/40/17):
 
Current S54 - 3.944 Final Drive
1st - 3.285 - 80kmh
2nd - 1.96 - 135kmh
3rd - 1.322 - 200kmh

This is roughly in line with your first quoted figure of 90 in first.

Turbo Gearbox - 4.285 Final Drive
1st - 3.23 - 75kmh
2nd - 1.913 - 127kmh
3rd - 1.258 - 194kmh
 
Still much higher than the s2000 gearbox's top speed of 106kmh for 2nd. Remember, that's a 6 speed box though. I did a quick google and found a link of all K series gearboxes and their ratios. (http://img.photobucket.com/albums/v504/MJ23FE/Tranny%20Gear%20Calculations/K-SeriesTransmissionTablev3-28-08re.jpg)
Using the same tyre size and limiter, I used a K20A Y2M3:

Final Drive 4.764
1st - 3.266 - 67kmh
2nd - 2.13 - 103kmh
3rd - 1.517 - 144kmh

From memory, I think you can buy adapter plates to put a K series box on a F20C. Currently it is looking to be your best bet
 
As far as I am aware, no one makes a custom final drive off the shelf. Hope this helps!
 

If you wanted to keep the current gearbox, to save on ****ing about, i believe someone 3 or so years ago sent an s54 final drive assembly to PAR engineering to have some 4.866 final drives made up. Maybe email them to see if they can make onr up for you. It would probably cost around 1-1100 to have made, and then there is the cost of fitting it. But, an e series swap is not great, its heavy and slow shifting, and you need a box, linkages, shafts. A k series box has you up for an adapter plate, box, custom shafts (byp could probably point you in the right direction)

So either way your probably a few thousand in, id freshen up the s54, fit an lsd and a 4.866 final drive and that would be the best bang for buck.

Hey Guys, I'm back again having done a fair bit more research.
 
PAR Engineering can indeed fabricate the gearing I need (any ratio I want) but at a cost of $1980.
 
My research has indicated that the "E" series boxes seem to be compatible with the "S" series boxes, and I have found some mention on US sites where they reference the use of the final gearing from the E250F box (4.933) but can't seem to confirm it's compatibility with the S54?
 
Does anyone have any insight or can point me in the right direction?
 
Thanks.

I'm finding a lot of chatter on Celica and Rav4 forums both in the US and UK and swapping "E" series diffs into the S54 seems to be fairly common, I can find plenty of references of the E250F option but can't find anything substantial.
 
If I can't find someone who can say a definite yes or no then I will buy one of each and have a play, though this is not the most ideal scenario.  (still better than forking out almost $2000 just for the gears, which will be a last resort)