I just checked my T5 to verify the gear ratios and I think this thread needs to be updated.
If I follow the instructions above, when I am in 5th gear the output shaft (the shaft connected to driveshaft) will turn more than 1 time for the 1 (360 degree revolution) of the input shaft (the shaft connected to engine).
This indicates that the output shaft (the shaft connected to the driveshaft end) must be the one turned the 1 (360 degree) revolution-NOT the input shaft (the shaft connected to the engine) so that you can easily see the gear ratio.
This is in line with general gear formulas for gear ratios which are stated generally as driven gear revolution/s divided by one drive gear revolution.
This means the transmission input shaft should be considered the driven shaft (motor drives it) and the transmission output shaft should be considered the drive shaft (as it is commonly called)
So the correct formula for determining the gearing of a T5 transmission can be found by the number of input shaft revolutions (driven shaft) divided by one output shaft (driveshaft) revolution.
Since it is generally accepted that gear ratios are based off of a 1 in the denominator, a 3.8 gear ratio is actually saying 3.8 revolutions divided by 1 revolution. What revolutions are we talking about? We are saying that
the 3.8 to 1 ratio is 3.8 revolutions of the input shaft (driven) in the numerator divided by the 1 revolution of the output shaft (drive) in the denominator of the ratio.
So, for my transmission in first gear:
If I turn the output shaft 1 time around in first gear, my input shaft rotates about 3.8 times around indicting my expected gear ratio for first gear.
Looking at it the other way:
Gears are devices to lever mechnical advantage through ratios. If you spin the input shaft about 3.8 times with the engine (in first gear) you will get only 1 full turn of the output shaft. You have gained 3.8 times the mechanical advantage from the transmission to get the car going from a stop by putting it in first gear.
Now let's look at my overdrive situation:
If I turn the output shaft 1 revolution in fifth gear (overdrive), the input shaft only turns about .7 times around. This is in line with my expectation of a .72 5th gear overdrive ratio.
If you spin the input about .7 times in fifth gear overdrive you will get one full revolution at the output shaft indicating and over drive condition.
As the formula says about three quarters of the way around on the driven shaft (lets call it .7 of the way around) divided by the 1 full revolution I get from the output driveshaft gives you your .7 to 1 ratio.
To clarify the overdrive, if you are in 4rth gear, for every 1 revolution of the input shaft you will get exactly 1 revolution of the output shaft, you are getting no overdrive. This is a 1 to 1 ratio.
But when you go to fifth gear we know that each .7 partial revolution of the input shaft gives you a full revolution of the output shaft. So in 5th gear, you spun the input shaft less than you did when you were in 4rth gear but you still got the full revolution from the output shaft. So you are in an overdrive condition-less engine/input shaft revolutions when in 5th gear to get the same distance traveled as in 4rth gear.
