If there is an exhaust leak near the O2 sensor, it could cause an affect on it's reading. Regardless, if there is an exhaust leak, it should be fixed. Broken exhaust studs were common on these engines, so check for a leak at the manifold gasket as well as the rest of the exhuast system. If a tune-up hasn't been performed in the last 48000km, then it would be a good idea to do so. Air filter inspection/replacement is considered part of a tune-up. If replacing ignition componants, like spark plug, wires, etc., stick with genuine Nissan or NGK parts. Use a fuel pressure gauge to check for excessive fuel pressure. If it is high, check the vacuum hose to the fuel pressure regulator at the back of the engine.
Higher tire pressure will help with the gas mileage, but will ruin your tires. Factory tire pressure spec. is 26 PSI and should be used for best tire wear, along with keeping the steering aligned.
More than that I can't advise without actually seeing the vehicle or knowing more about it.