Your modified VX is suffering from the classic performance=lower mileage syndrome. Anything you do to boost performance eats away at OEM equipped mileage. My wife and I drive stock VX '99's daily and get 20mpg aveage, no problem inspite of 10 % ethanoyl. Yes, we use Lucas gas treatment to compensate for the ethanoyl mileage penalty. Go OEM and your problem will be alleviated, it's that simple.