Leaf Spring Sag
from John Twist of University Motors Ltd.

The MGB should sit horizontally. If you measure from the bottom of the chrome strip to the floor, through the center of each wheel, you should have the same number, plus or minus 1/2 inch, at the LF, RH, LR, and RR. As the leaf springs collapse, the Left Rear usually fails first (on LHD cars) which not only lowers the left rear but lifts the right front. The car should measure the same fore and aft as well as left and right.

Measurements made off the car are not reliable as the spring rate can be different from spring to spring.