A good place to start is by making sure your car is as aerodynamic as possible. Open windows and roof racks, for instance, block the air flowing around the car. The engine has to work a bit harder to push the car along, which worsens fuel economy.
Keeping the tyres pumped up to the correct pressure is vital, too. A tyre with low pressure squashes out, creating a larger ‘contact patch’ with the road. That creates more friction than normal which the engine has to work harder to overcome, worsening fuel economy.
It’s worth noting that the bigger a car’s wheels, the worse its fuel economy will be. A high-spec car with 20-inch wheels may look great, but its fuel economy is often several MPG worse than a lower-spec model with 17-inch wheels because the engine has to work harder to turn the larger wheels.
Your car’s electrical equipment uses power generated by the engine. The more of that equipment you have turned on, the harder the engine has to work, so the worse your fuel economy will be. The air con, in particular, can have a big effect. Keeping unnecessary equipment turned off will improve fuel economy.
But by far the best thing you can do to make sure your car does as many MPG as possible is to have it serviced regularly. If your car’s engine isn’t fit and healthy, it simply won’t be able to give you its best MPG.