Q: Leaking coolant and poor gas mileage.

Some key things to check to ensure your vehicle gets correct gas mileage would be your air filter, fuel filter, quality of fuel, and oxygen sensor integrity. A dirty air filter will suffocate your engine causing bad gas mileage. Dirty fuel entering the engine caused by a dirty fuel filter will cause the engine not to produce as much power thus travel less miles in exchange for the same amount of fuel. The same goes for bad gas itself. If your oxygen sensors are malfunctioning this can cause your engine to run rich and thus decrease your fuel mileage. I would recommend having a professional come to inspect your vehicle to see which of these is at fault.