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More news about the gears.
The MKII could have several gearing systems inside. You need to pull the output gear to find out what you have in yours.
The two gears offered inside the transmission that drive the output gear are 10 or 11 teeth shaped like a worm gear. There are different part numbers for the #10 gear in the supras but only one number for the #11.
There should be 2 numbers on the bottom of the plastic output gear such as 11 on top and 33 on the bottom of the bottom, or 10 on the top and 31 on the bottom of the bottom.
If yours says 11 33 then your screwed as far as changing the output gear.
The 11 33 is the same as the R154 transmission and its the only combo offered anywhere in the world.
The 10 and # offers more choices. There are combos of 10 35, 10 32, 10 31 for the 87-92 supra for the W58 twincam and for the Celica-Supra the choices are 11 33, 10 32, 10 31 for the W58 transmission and earlier 81-83 models had a 6 21 but Im not sure which transmission that is for.
If your is the 11 33 then the only option is to have a shop change the gear inside to a 10 that coincides with the matching output gear and see which one works for you or go to a speedo calibration shop and maybe they can put a ratio changer outside the transmission and change it that way.
Part numbers for the different items are as follows:

Plastic output gears with #10 metal gear combos.

Plastic output gear Metal drive gear

#31 33403-29145 #10 33481-22030
#32 33403-29125 #10 33481-22030
#35 33403-19235 #10 33481-14031


plastic output gear for #11 metal gear

#33 33403-29115 #11 33481-14071 (R154 8808-9305)

I just had a shop install the #11 with the 33 plastic in my W58 and my speedo is reading a little fast. I am going to do side by side comparisons with another car to see how far off it is but Im guessing about 7-8 mph fast.
I have the 3.91 LHD on my car and am running 225 55 ZR 16 tires. My odometer is fast about 2/10ths in 4 miles.