[Big hammer]

10 mpg ? I would consider 15 mpg on the low end. What rear end gears do you have? Are you checking tire pressure at 35 psi? Does your speedometer match a gps app in a smart phone? What do the spark plugs look like? 90 rpms idle sounds like late timing. My tail pipe burns a light gray color. Salem Or are you driving I'm mountains, a HC head would be helpful with power and fuel mileage !