Traveling by car is the easiest way to get around Johannesburg, as the city's public transportation is not that reliable or extensive, though this is changing (the Gautrain, for example, is incredibly reliable). The general speed limit for city streets is 60 kph (37 mph); for main streets it's often 80 kph (50 mph), and for highways it's 100 kph or 120 kph (62 mph or 75 mph). But be warned that Johannesburg drivers are known as the most aggressive in the country, and minibus taxis are famous for ignoring the rules of the road, often stopping for passengers with little or no warning. Most city roads and main countryside roads are in good condition, with plenty of signage. City street names are sometimes visible only on the curb, however. Avoid driving in rush hours, 7 to 8:30 am and 4 to 6:30 pm, as the main roads become terribly congested. Gas stations are plentiful in most areas. (Don't pump your own gas, though; stations employ operators to do that for you.)
Almost everywhere there are security guards who look after parked cars. It's customary to give these guards a small tip (R2 or R5) when you return to your car. Most big shopping centers have parking garages (about R8 per hour).
If you plan to drive yourself around, get a good map of the city center and northern suburbs or buy or rent a GPS (available at the airport and most car-rental agencies). MapStudio prints excellent complete street guides, available at bookstores and many gas stations and convenience stores.
Major rental agencies have offices in the northern suburbs and at the airport.