Ford Mustang Owner's Manual page 171

If you do not know the actual weights of these
items, use the following procedure to figure the
weight of a load:
1. Weigh your vehicle without the driver,
passengers and luggage. You might take
your vehicle to a shipping company or an
inspection station for trucks.
2. Then weigh your vehicle with the driver,
passengers and luggage.
3. Subtract the first reading from the second to
figure the total weight of the load.
After determining this load, you must also make
sure that the total weight of your vehicle, plus
the load it carries, is less than its GVWR. Also,
make sure that the weight your vehicle carries
over each axle is less than the GAWR for the
respective axle.
If your vehicle exceeds the GVWR, remove cargo
from your vehicle accordingly. If your vehicle
exceeds the GAWR for either axle, shift the load
or remove cargo accordingly.
Improper distribution of your vehicle's
load or carrying weight in excess of the
amount shown on the tire decal can cause
rapid tire wear or loss of vehicle control.
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