Easy one
Sure grip.. Jack up the back of the car so both wheels are off the ground. Turn a rear tire around until the valve stem is at the top, pointing straight up. Look underneath the car at the rear U-joint and take note at the position the caps are facing. Rookies can mark with a sharpie or put a sticker on the cap facing you. Slowly start turning the rear wheel as you look at the U joint and count how many times the cap with the sticker goes by. Stop counting when the Valve stem on the wheel goes back to the top, one full rotation around the wheel.
3 1/4 turns of the U-joint is 3:23 gears. 3 1/2 is 3:56. Almost 4 turns is 3:90 ect
Non Sure grip units are a little trickier.
Jack up only one wheel of the car with the other wheel firmly on the ground and start counting the same way as before but do two full rotations of the wheel to get the right number.
The most common by far is the 3:23 but you might have the 3:56 if no one has messed with it
