Note: most of my info at hand relates to Euro and USA spec vehicles. Will update this later.

There are two differences between the SR and GT spec gearboxs that I know of.
1. The bellhousing is different. Clutch on non exhaust side. One says "4AG" on the top, other says"3A".

2. The speedo gears are different according to the diff ratio and tyres it had.

Cars with the 4A-C mostly had the 3.909:1 diff (3A-U has 3.727:1). They have 185/70R13 tyres or 185/65R13 tyres or similar. And the speedo gears in the gearbox are 20:6.

Cars with the 4A-GE mostly had the 4.3:1 diff. Many had 185/70R13 tyres, some had 14" wheels with 185/60R14 tyres, and 195/60R14 was an option. The speedo gears are most commonly 22:6 ratio (suits 185/70R13 and 195/60R14 best), or can be 19:5 to suit slightly smaller 185/60/14 tyres.

The AE86 speedo cluster is 637 REV = 1KM
This is written on the front of the speedo, seen when you take the clear cover off.


Changing to factory spec speedo reading:
You take your speedo drive out. You have a 20 tooth gear and want a 22 tooth one (or 19). Some KE70s have what you need. Otherwise you can just order the gear and shaft from Toyota.
If you want the 19:5 ratio, take the shifter housing off the gearbox. Replace the metal speedo drive ring from a 6 tooth to a 5 tooth. Its a worm gear, when the shaft spins once the plastic gear turns 5 teeth. Im not sure how common the 5 tooth worm is, but its only found on some AE86 gearboxes. You may have to order it.

Plastic gear on shaft
33403-19135 N=19
33403-19155 N=20
33403-19165 N=21 (not specificly a AE86 part)
33403-19175 N=22

Metal worm gear on output shaft
33481-12030 N=5
33481-14040 N=6


Mix and match
The parts listed are all the same diameters. You can mix and match ratios, you don't have to use a matched set. You can use the plastic gear and shaft from a KE70, but you can't use the metal worm gear from a KE70. You can also find a plastic 22 gear in most KE30s, but may have to press in onto your shaft. The speedo gears from a TA22 and 1979 TE72 will not fit as the output shaft is smaller and the gears are different diameters. From about 1980 onwards (bigger output shaft), the T-18 will have compatable speedo gears, although are probably 20:6 same as AE86 anyway. With the early gearbox. the worm gear slips on and is located with a ball. With the late gearbox (all AE86) the worm gear has its own spline.