Century 974W25 Installation, Operation, Maintenance, Parts page 21

Section I - SAFETY PRECAUTIONS (cont'd)
SAFE TOWING
To use the formula, multiply the unladen weight at the front axle by .6.
Multiply the result by the wheel bases. Then divide that result by the
overhang. So, you should calculate the maximum lifted load for each tow
truck, using this formula, post those limits in the truck and instruct each
driver to strictly observe those limits.
You should also observe gross vehicle weight ratings (GVWR), gross
axle weight ratings (GAWR), and the towing equipment ratings.
Unladen weights at front and rear
Wheel base and overhang distances for tow slings and wheel lifts.
Wheel base and overhang distances for truck
I-15