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 |
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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!
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