Club Car's owner's manual states:
Avoid driving the vehicle on slopes exceeding 20% incline.

My explanation:
A 20 degree incline angle is close to a 37% grade so the answer is 20% not 20 degrees.
Incline is also called pitch or rise and is a ratio of rise divided by run.
The percentage of incline is rise/run x 100.

Here is a example of incline:


Here are some inclines vs percentages:

Originally posted by dougmcp
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