I recommend using the multi-functional display at the bottom right of the race HUD and go to the fuel mix option. Adjust the fuel to a leaner mix. You will lose more and more power the leaner you go, so keep that in mind.
Lifting and coasting before corners (getting off the gas a moment before braking for a corner) also works for saving fuel. If you are using a manual transmission, short shifting (shifting gears earlier than normal) works too.