Easy method to identify rear gear ratio

[Guest norm[T12SDSUD]]

Ok, this is a quick and accurate method for determing your current rear gear ratio on non LSD rear ends.

 

Jack up the rear passenger side of your car as the drivers side tire must not turn at all in order for the test to be accurate.

Mark a place on the rear passenger tire tread with white out and mark a place on the ground for reference and line both marks up.

 

With both marks perfectly lined up , climb underneath the car and make a Horizontal two inch long mark on the passenger side of the driveshaft and make sure it is centered like the diagram below.

 

 

UP

___________________________________

 

 

____ driveshaft mark

 

 

___________________________________

 

DOWN

 

This method will work because with the drivers side wheel secure, the internal differential action will allow one full turn of the passenger wheel to cut the normal pinion rotation in half.

 

To find the gear ratio do the following:

 

Place the transmission in NEUTRAL and block the drivers side wheel to prevent the car from rolling if you are not on perfectly level ground.

 

Turn your passenger tire exactly one full revolution CLOCKWISE so that the mark on the tire and the mark on the ground line up perfectly.

 

Now climb underneath the car and look at the driveshaft.

 

If the mark you made in the center of the driveshaft now faces the ground, then you have a 3.54 rear gear. The driveshaft will have turned 1.77 turns.

1.77 X 2 = 3.54

 

 

If the mark you made in the center of the driveshaft is almost at the exact same place as where it started then you have a 3.9 rear gear. The driveshaft will have turned 1.95 turns.

1.95 X 2 = 3.9

 

If the mark you made in the center of the driveshaft is now in between a position facing the ground and the original starting position then you have a 3.7 rear gear. The driveshaft will have turned 1.85 turns.

1.85 X 2 = 3.7

 

If the mark you made in the center of the driveshaft is above its starting position then you have a 4.11 rear gear. The driveshaft will have turned 2.055 turns.

2.055 X 2 = 4.11

 

 

If the mark you made in the center of the driveshaft is now on the drivers side of the car then you have a 3.36 rear gear. The driveshaft will have turned 1.68 turns.

1.68 X 2 = 3.36

 

If you want to doublecheck your readings then turn the passenger wheel COUNTERCLOCKWISE to the original mark and then recheck by turning the passenger wheel CLOCKWISE.

 

 

If the differential is out of the car then scribe a line on the passenger side of the driveshaft mating flange so that it is perfectly centered and then without moving the driveshaft mating flange make a mark on the passenger side halfshaft mating flange and another on the differential casing so that these two marks line up perfectly.

Prevent the drivers side halfshaft flange from moving and then turn the passenger side halfshaft flange exactly one full rotation CLOCKWISE.

 

The resultant position of the driveshaft mating flange mark is the same as described above for the driveshaft mark and should be used to determine the gear ratio of the differential.

 

later,norm