Speculate to accumulate mate! If it's been a while, give it a good service. Run some decent fuel system cleaner through it, check tyre pressures are correct, and bin the sh!t cone filter! Check all vacuum lines for splits, breather hoses are solid and not spongy, clean out the throttle body, and get it on vagcom just in case. Make sure your water temp is reaching 90degs within 5-8 minutes and staing there too. All this should make a difference to fuel economy, but you will obviously have to spend to see benefits, not counting the cleaning of tb etc etc