What do variable rate springs do?
These springs increase load carrying capacity (usually 1,000 to 1,500 pounds.) without increasing ride harshness. Heavy-duty replacement springs will also increase load carrying capacity, but their stiffer spring rate may make the ride too harsh for comfortable everyday driving.
Consequently, variable rate springs are a better option for the customer who occasionally pulls a trailer, carries extra passengers (car pool commuters), carries extra loads (like an anvil salesman), or uses his vehicle for occasional weekend off-road recreation.
If original springs are sagging and need to be replaced anyway, upgrading to variable rate springs is an option well worth considering. They cost about the same as standard replacement springs and are no more difficult to install.