[fulltiming]

Here is the wording from the Arizona Motor Vehicle Division in their Vendor Brochure.

What is the rate of tax on use fuel?
Use fuel (diesel) in Arizona is taxed at two levels:
If the use fuel is used in the propulsion of a use class motor
vehicle on a highway in this state, the tax rate is $.26 for
each gallon.
1. A "use class motor vehicle" means a motor vehicle that uses use
fuel on a highway in this state and that is a road tractor, truck
tractor, truck or passenger carrying vehicle having a declared
gross vehicle weight of more than twenty-six thousand pounds or
having more than two axles.
2. If the use fuel is used in the propulsion of a light class motor
vehicle (such as a pickup truck), or an exempt vehicle (certain
school buses, government and non-profit vehicles – see ARS 28-
5432.C ) on a highway in this state, the use fuel is taxed at the
same rate per gallon as the motor vehicle fuel tax ($.18 per
gallon).
(See ARS 28-5601, 5605, 5606).

http://www.azdot.gov/mvd/fueltaxevas...orbrochure.pdf