Porsche 911 Carrera Manual page 186

Uniform Tire Quality Grading
Quality grades can be found where applicable on
the tire sidewall between tread shoulder and max-
imum section width.
186
Practical Tips, Emergency Service
Example
All passenger car tires must conform to Federal
Safety Requirements in addition to these grades.
Treadwear
The treadwear grade is a comparative rating
based on the wear rate of the tire when tested un-
der controlled conditions on a specific govern-
ment test course.
For example, a tire graded 150 would wear one
and a half (1-1/2) times as well on the government
course as a tire graded 100.
The relative performance of tires depends upon
the actual conditions of their use, however, and
may depart significantly from the norm due to var-
iations in driving habits, service practices and dif-
ferences in road characteristics and climate.