Yank it off and shake it, if it rattles when you shake it then that's a good sign. While it's off, clean it (wipe it off, oil doesn't taste good) and blow in both ends, it should flow freely from one end and plug from the other, sludge and varnish can make it not seal all the way and rob your power and mileage because of a massive vacuum leak that it causes.

Things that affect mileage and power:
Gear ratio in the axles
4x2 vs 4x4
Ignition system condition
Vacuum leaks
Upstream o2 sensor condition
Enthusiasm with the go pedal
Tire size
Tire size that's told to the tuner
Toting unnecessary weight like a toolbox full of junk
A dinged up hood and no grille acting like a sail

I used to get "13.5 - 14" mpg until I plugged in the tuner. There is an option for tire size in the tuner and from the factory (in the PCM) it was set for 29" tires, I changed it to 32.5" tires and my mileage dropped to 11.5mpg. If I switch it back to 29" tires in the tuner then I'll get the 14mpg lie from the odometer again.
The way I understand it (and I may be wrong) is the odometer gets a reading from the speed sensor on the rear differential and uses the revolutions of the ring gear along with how tall the PCM is told the tires are to calculate miles driven.
Smaller tires have more revolutions per mile rolled so your mileage will show higher than actual if the computer thinks that the ring gear is turning shorter tires than it is.