Quick gear ratio question

[Mick_D]

7:31 is the desirable fifth gear ratio, right?

[chris_seven]

7:31 is the ring and pinion tooth count.

If you fitted a 5th gear with a 1:1 ratio it would be the overall gearing in that gear.

[stownsen914]

Originally Posted by Mick_D

7:31 is the desirable fifth gear ratio, right?

Depends entirely on the specs of the car and the intended usage. If you are referring to putting a 7:31 in a later 915 trans that came with a 8:31, you will of course getting shorter gearing, which many consider desirable from a performance standpoint.

[puddy]

I have an 86 915 in my turbo hotrod with the 8:31 ring & pinion and couldn't imagine having a shorter 1st gear, its really short.

[kent olsen]

If you had a normally aspirated engine with that 8:31 you would think differently

[Bill Verburg]

all 911 tranaxles transmit power through 2 pairs of gear sets
1 pair is the cwp that turns the power 90° from longitudinal(engine and the 2 transmission main shafts) to transverse(the 2 axle shafts connected to the wheels
here is a 915, the pic is upside down from as installed in a car but it shows the longitudinal input shaft 9(bottom), longitudinal lay shaft(top), 4 gear sets, pinion shaft the p in cwp) to the left


again upside down, bottom is the longitudinal input from the clutch(which would be to the left), at right is the pinion end of the output or lay shaft, the hole in the back is for an axle shaft stub.


The crown wheel(c in cwp) is bolted to the differential, here a ZF 40% lsd, the c and p mesh to turn the power delivery 90°



there are 2 widely used cwp gear ratios, older 915 through ~'74 used 7:31 later 915 from ~'75 on used 8:31, *:31 is stronger

a 5 speed 915 will have 5 pairs of meshed gears, 4 are shown above, these have their own ratios
for example a '79 SC w/ 915/61 or /63 trans will have 8:31 cwp and 11:35 1st, 18:33 2nd, 23:29 3rd, 26:26 4th, 23:28 5th

since each gear uses its' own ration you need to convert the ratios to
8:31=3.785, 11:35=3.1818, 18:33=1.833 etc
the overall gear ration is the product of the cwp and individual gear ratio
1st is 3.875 * 3.1818 = 12.330

but that;'s not the end of it because the tire height further alters the final gearing, the shorter the drive tire the lower the overall ratio the taller the higher, 1" of tire OD delta correlates to ~4% delta in gearing

[Mick_D]