Definitions:
Spring
Rate is half the
difference between the loads 1 inch above and 1 inch below a specified
position. Another definition would be: The amount of force it takes to
compress the spring 1inch and is expressed in ld/in. The
lower the rate, the softer the spring.
If the front of your truck is sagging you need more spring load, not
more spring rate.
Spring
Load is the amount of weight the spring is designed to carry at
a certain height. This is also called the Design Load or Load Rate.
Think weight carrying capacity.
Load
Rate
is not to be confused with Spring Rate. Load
Rate is the amount of weight a spring is designed to carry at a certain
height.
Let's
say a spring has:

A
rate of 200 lb/in

Designed
for a 3Inch deflection

When
deflected 3inches the spring is supporting 600 lbs

Therefore,
the spring has a Load Rate of 600 lbs. Not a Spring Rate of 600
lbs.
Unsprung
Weight is the weight of the tires, wheels, knuckles, hubs, axles, and
half the weight of the springs, shocks, control arms, and/or links.
Sprung
Weight is the weight of the body, chassis, drivetrain, tools, parts
and the other half of the total weight of the springs, shocks, control
arms and/or links.
Wheel
Rate is the spring rate actually measured at the wheel (or tire).
The wheel rate is usually lower than the true spring rate due to factors
such as spring position and control arm or axle leverage that can
effectively lessen the spring rate at the wheel versus the actual spring
rate at the spring. If you move the spring closer to the tire (and
the spring travels parallel to the wheel), the wheel rate and spring rate
will become almost the same.
Variable
Rate Springs have a soft initial spring rate and to absorb the subtle
irregularities of the road/trail progressing to a firmer rate to handle
large bumps. These springs increase in rate as they are compressed.
Effect
On Rate:
In
the formula below you will see Number of Leaves, Leaf Length, Leaf Width,
and Leaf Thickness. Here is how they effect rate:
Dimension
Increased 
Effect
On Rate 
Number of
Leaves 
Higher 
Leaf Length 
Lower 
Leaf Width 
Higher 
Leaf
Thickness 
Higher 
