RPM's

Sounds like you've found it to me , fill it up drive it for an hour then refill and do the math on your mileage . I would do it twice once around town and once down the highway , try not jumping on it during the highway run just find an on ramp gas station and go 1/2 hour from full there to an exit then loop back to the same spot refill and do the math , I bet you get at least 12 mpg .