this is kind of a gray area. Terrain type (IE lots of hills) can affect fuel economy in different ways. 14 would be great with a heavier trailer. 8500 isnt huge, but that is a pretty substantial trailer. 10 is not terrible, although i would expect a minimum of 12 generally towing something around that wieght. What kind of economy are you getting unloaded??? and does it seem to be running well........and how many miles were on that old fuel filter????