Spring Rate vs Wheel Rate
Spring rate is the amount of force it takes to compress a spring 1-inch. Wheel rate is the spring rate actually measured at the wheel. Both are expressed in lb/in.
In order to calculate wheel rate, you need to know the spring rate, the motion ratio and the spring angle, as shown above. Then you can solve the following equation.
When a spring is mounted in its stock position on the front of a 1979-2004 Mustang (shown on the right side, above) the motion ratio is about 0.50, and the wheel rate is about 0.25 x the spring rate. That is, a 600 lb/in spring produces a wheel rate of about 150 lb/in.
If coilover springs are installed (shown left side, above) the spring is moved close to the wheel, the motion ratio becomes about 0.95, and the wheel rate is about 0.90 x the spring rate. A much lighter 167 lb/in spring produces the same 150 lb/in wheel rate.
The chart below provides motion ratios and wheel rate factors that can be used to calculate an approximate wheel rate for a spring installed on a 1979-2004 Mustang. To obtain the wheel rate, multiply the spring rate by the factor that corresponds to the spring type and position on the car (stock or coilover, front or rear).
Note: These numbers are approximations and this discussion omits some variables in the interest of simplicity.