I'm getting less than 10 mile per gallon and the service engine light stays on and the cruise control doesn't work

Wow that is quite the drop. You need to start with a profession scan of the computer. Only with the code or codes can we begin to diagnose what is going on here. It could be a very bad misfire so I wouldn't drive it until you get it checked out. Here is a link to find a shop near you. https://repairpal.com/repair-shops Alex

Look to the right of this page and click on this>>Common 2006 Subaru Forester problems....there are a few problems and one or more may be your problem....also,see answer #1....

Just as a side note: *Anytime* the check engine light remains on while vehicle is being operated - the cruise control functional - by design - is rendered inoperative. When the engine performance problems are corrected - cruise control will return to normal.