Ford Mustang Owner's Manual page 170

You can find your vehicle's GVWR and GAWR
on the Safety Compliance Certification Label on
the left front door lock facing or the door latch
post pillar.
The weight limits of your vehicle's tires affect
the GVWR or GAWR limitations. Usage of
replacement tires with higher weight limits than
originals does not increase GVWR or GAWR.
Usage of lower capacity replacement tires may
lower GVWR and GAWR limitations.
Do not exceed the GVWR or the GAWR
specified on the Safety Compliance
Certification Label.
Limiting Your Vehicle's Load
The load that you add to your vehicle must not
exceed the maximum load stated on the Safety
Compliance Certification Decal attached to the
left front door pillar.
To figure the weight of your vehicle's load, add
the weights of:
the driver and passengers
luggage and any other items that you put in
your vehicle
the tongue load of the loaded trailer
the weight of any equipment that has been
added to your vehicle since it was delivered
to your dealer
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