A simple free way to extend your miles per gallon is to make sure your tyres are inflated correctly.
Just because they look inflated doesn’t mean they are at the correct pressure. Seeing our Yaris MPG slip to 48mpg I checked the tyres, 1 was correct the others were down to 20psi. Topping up the pressure got the mpg back up to 55mpg. That’s like a 10% improvement! or the equivalent of a 14p saving per litre of fuel!
The same can be true when replacing your fuel filter. But that costs money.