Measure the width - [1 1/2", 1 3/4", 2", 2 1/2", 3", Other]
[D] Total pad thickeness at centerbolts (all leaves)
[C] Measure the free arch. hold a straight edge through the centerline of the spring and measure from this line to the top of the main leaf next to the centerbolt
[A, B] Measure the 'A' and 'B' lenght of the main leaf spring (longest)
Identify the end type of your spring (standars, berlin, reversed, square cut, square cut w/ point, or tapered)
where i borrowed this from 