Having a clean windscreen saves fuel, it achieves this by improving the air flow
around the vehicle while the cars in motion, Improving the cars aerodynamics reduceses the air resistance there for requiring less fuel to move. Another tip to save fuel is to make sure that the tyres are at the correct pressure. If they are not and are to flat for example this would increase the surface area of the tyre on the road which in turn creates more friction so require more fuel to move the vehicle. So having the correct air pressure will reduce the surface area of the tyre that impacts on the road reducing the amount of fuel to move.