You'll need a car to get around Tucson and the surrounding area and it makes sense to rent at the airport; all the major car-rental agencies are represented. Driving time from the airport to the center of town varies, but it's usually less than a half hour; add 15 minutes to any destination during rush hours (7:30 AM-9 AM and 4:30 PM-6 PM). Parking is not a problem in most parts of town, except near the university, where there are several pay lots.
If you haven't rented a car at the Tucson International Airport, or if you want to save the 10% "airport concession fee" the airport imposes on renters there, several car-rental agencies have pick-up and drop-off locations in the central, northwest, and east areas of town. Carefree Rent-a-Car, a local company, rents reliable used cars at good rates. If you think you might be interested in driving farther into Mexico than Nogales (where you can park on the U.S. side of the border), check in advance to make sure that the rental company will allow this and that you are covered on your rental-insurance policy: many rental-insurance agreements do not cover accidents or thefts that occur outside the United States.
From Phoenix, 111 mi northwest, I-10 east is the road that will take you to Tucson. Also a major north-south traffic artery along the west side of town, I-10 has well-marked exits all along the route. At Casa Grande, 70 mi north of Tucson, I-8 connects with I-10, bringing travelers into the area from Yuma and San Diego. From Nogales, 63 mi south on the Mexican border, take I-19 into Tucson.
Carefree Rent-a-Car (520/790-2655)