noob needs help with mileage - only 10 MPG!

13 is still lower than it ought to be. Something's still the matter.

One question: have you checked whether your speedometer and (more importantly) your odometer are reading correctly? If you've installed larger tires or a numerically lower rear axle without changing the speedo drive pinion, you'll record fewer miles than you've actually travelled. And have you checked carefully for fuel leaks?