When it gets too hot, your engine could overheat. Avoid items on the roof. A lot of weight on the top of the car will destroy the aerodynamics. Aerodynamics is how the vehicle passes more efficiently through the air. Lighter aerodynamics is an easy way to get more out of your gas mileage.
More weight ruins gas mileage.
If your vehicle is at around 6,000 or 7,000 miles, a common problem is the breaking of the timing chain or timing belt. Inspection of the timing belt is very important for a number of reasons. For one thing, the belt runs the valves of the engine. This is what allows the fuel to come in and out. If they break, it is not only very inconvenient in terms of time and money to get it fixed, but you could do a lot of internal damage to your engine.