Anybody know a way to tune the engine use less petrol.

What is your question?

My love to my Jag.xk8 are not to break, but the use of petrol, gasoline are devastation. I live in Europe and that make the petrol price in top of the world!! My Jag. use around 17 Lit/100 km minimum and top 20+ lit./ 100 km. on freeway. Anybody who may know any smart way to reduce the gasoline use, without to damage the engine or slow it down. I'm not a racing driver, too old for that kind of fun.