There are a number of ways of identifying a differential - either by counting the crownwheel teeth and dividing by the number of pinion teeth (eg c/wheel 35 pinion 9 = 35 / 9 = 3.89), or by counting the number of turns the front flange requires to turn both output flanges by 360 degrees (or one output flange by 720 degrees) - just more than 3.6 means 3.63, just more than 4 means 4.11.
By just looking at a differential, you can often narrow down the ratio. For convienience, we often split differentials into 4 types:
- Type A Square front pinion flange. 5/16 bolt holes all round (1/2 AF spanner size). Small quarter shaft diameter inside. Casing no's G, Y, GA, FC to 120,000.
- Type B Square front pinion flange. 5/16 bolt holes all round (1/2 AF spanner size). Big quarter shaft diameter inside. Casing no's GE, FC from 120,001.
- Type C Square front pinion flange. 5/16 bolt holes on front flange (1/2 AF spanner size). 3/8 bolt holes on ourput flanges (9/16 AF spanner size). Casing no HB.
- Type D Round front pinion flange. 3/8 bolt holes all round (9/16 AF spanner size). Casing no's FH, FK, FR, FD, HC, KC, KD.
Type A and B are indistinguishable from the outside, but Type A shouldn't be fitted to a 13/60 as the quarter shafts are liable to break. Type C (which was only used for the Vitesse 6 originally) is very prone to quarter shaft failiure.
Having decided which differential you have, you may now wish to alter the ratio. The following table gives rounded figures for miles/kilometersper 100 rpm.
MPH/1000 IN 4TH GEAR
KPH/1000 IN 4TH GEAR
MPH/1000 IN OVERDRIVE TOP
KPH/1000 IN OVERDRIVE TOP
Bigger wheels, non-standard tyres etc, will affect the above figures.