Determining Gear Ratio

The "problem" with the W58 transmission is that 3 different versions were used in the '82-'85 Celica/Supras that gave 3 different gear ratios. To my knowledge there's no markings on the transmission case itself to tell you which one you've got.  It is marked on an ID plate that mounted on the car so unless you're pulling the tranny from the car yourself, you'll have to determine the ratio on the bench. The chart below shows the various ratios and the highlighted column is the ratio I have.

So what does this mean in terms of reduced engine "stress". A quick Goggle found a number of sites that provided a calculator to do MPH versus RPM calculations based on: Tire Size, Differential Ratio & Transmission Gear Ratio. The chart below shows a comparison (5th gear vs 4th gear) of the Toyota 5 Speed versus the TR6 4 speed (without overdrive) & with their respective differential ratios. The stock TR6 has a 3.69:1 ratio while I'm running a 4.08:1 Nissan differential with the Toyota transmission. That ratio gives me increased performance while sacrificing lower RPMs when cruising down the highway. Despite the higher ratio rear end, the 5th gear gives me 10 MPH more at the same RPM level as the stock TR6 4th gear. Or another way to look at it is that at 70 MPH, the engine is turning about 500 RPMs less. If I had gone with the Nissan 3.54 ratio I would have gained another 500 RPM decrease at each speed.

So....how do you determine your ratio? It's actually quite easy when you have the tranny sitting on a bench. All you have to do is put it in reverse, mark to top of the input and output shafts, rotate the input shaft and count its revolutions to one revolution of the output shaft. In my case the input shaft rotated 3.8 times to 1 revolution of the output shaft. I found that the easiest way to do this was to slide the clutch plate on the input shaft and the flange on the output shaft. I rotated the clutch until one of the flange holes was at 12:00 and put a bolt through the hole. I then marked the 12:00 clutch position with a chalk mark. This made it very easy to see and count rotations. OK so it's only easy if you do it right........as you'll read below, it took me a while to "get it right"!

A chalk mark on the clutch and a bolt in the flange

As you can see from the chart, the spread across the 3 car transmissions is the greatest in reverse so it's easy to determine which one you have................or so it would seem. Reverse is located to the right and down when seated in the driver's seat. So I moved the gear shift "dog" to the right and down. Did the revolution counting and came up with 3.25 which would indicate the middle tranny according to the chart above. Then I put it in 5th gear, to the right and up, and counted revolutions again.............and got 1.9! Now that made no sense at all so I emailed some friends for advice. Al Gary emailed me back and suggested counting again........finding reverse is easy, he said, because the shafts will rotate in opposite directions............ opposite directions........yea right......of course they will.........that's why it's called REVERSE. So.....back to the basement to try again. I move the shift dog to the right and down for reverse but the shafts are going in the same direction! Do I have a busted tranny with no reverse? DUMB DUMB DUMB........and the light bulb in my brain finally turns on. As you can see in the picture above, I don't have a shift lever installed yet, so I'm just moving the shift dog to get it in gear. The shift dog is where the BOTTOM of the shift lever will sit soooooooooooooo.........when you move the TOP of the shift lever to the right and down, the BOTTOM of the shift lever is moving in the opposite direction or left and up. DUH. Once I figured that out, the shafts moved in opposite directions like they're supposed to.  In the gear ratio table, I've highlighted my transmission which has the best 5th gear ratio!

NEXT........Kit Contents