Gearbox Final Drive Modification
Some GT-Fours (for example the ST185RC and some US models) came with a 3.933 final drive ratio which gives, well, not exactly startling acceleration... However there is an affordable cure. Changing the final drive to 4.285 drops the overall gearing and you can replace that stuffed second gear synchro while you are at it :-)
All Japanese market GT-Fours (except the ST185RC) come with a 4.285 final drive ratio. 4.285 being around 9% lower than the stock ratio (3.933) will give better acceleration. However, it is not all good as your fuel economy will suffer dramatically. Although I have yet to test it there is minimal effect on maximum speed, as the different ratio allows you to utilise the power curve a bit better allowing you to rev higher in 5th.
An example of the advantage of the quicker gearing is a story told to me by a local rally driver who used to drive a Group A GT-Four RC (before it was rolled and destroyed in the Perth Rally... oops). They attempted the Motu stage of the Rally of New Zealand with the 3.933 gearing. They were forced to use 1st and 2nd all through this twisty stage and as a result ran out of fuel! They completed the stage after siphoning some from another competitor who had blown a gearbox. The next year they came back with the 4.285 ratio and did the stage in 2nd and 3rd, completing it without problems.
Anyway with the 4.285 ratio at 100kph I'm only doing around 2900rpm, quite bearable. If you are into 0-60mph sprints this may not be for you as the car hits the rev limiter at around 92kph, meaning two gear changes instead of one. At the local racetrack I've had the car doing 200kph with it still accelerating in 5th.
The good news is that the 4.285 parts are interchangeable with those of the 3.933 parts. You need three parts, pictured above:
Gear, Ring 41221-20240
Gear, Oil Pump 33407-20011
The other good news is that these parts are used in the AW11 MR2 (only in the Supercharger) and in the SW20 MR2 Turbo. This will mean that any Toyota dealer in the US can supply them. Some US GT-Fours may have the ratio also but this is unconfirmed, so check with your dealer.
When fitting these the only thing you will need to do is reshim the bearings of the output shaft, as per the manual description, no big deal. Obviously if buying a gearbox from a wrecker just for these gears you really want to know what you are buying. Unfortunately Toyota don't stamp the gearbox number on the casing, only a serial number which I couldn't decode.
Toyota also don't stamp the part numbers on the parts themselves. To tell if you have the 4.285 parts the ring gear will have 60 teeth, and the pinion 14 (60:14 = 4.285). In comparison the 3.933 ratio parts are 59:15.
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