Bigger rims and tires create more rotational mass, which usually leads to 1 or 2 mpg less. Also because of the larger diameter you are actually traveling further and faster than the speedo and odometer are indicating which could make up another 1 or 2 mpg loss. I had to buy a valve cover gasket set, and that fixed my leak. My started running weird once so i decided to change my sparkplugs. My oil wasnt overfilled but when took the first spark plug out it was covered in oil. So changed my valve cover gasket along with all the grommets and sparkplug hole rubber gaskets and it stopped leaking.