Or use the trip odo. I reset mine every time I tank up.
I also reset my MPG with every tank. I find mine to be very accurate.
Mods do not gain MPG. Everyone wishes they did so that they can justify adding power mods to their truck. In fact I would ditch that K&N filter. Intake an exhaust gain more power by allowing you to burn more gas.
OK not totally true, I would consider these 2 things replacements rather than mods: Switch all oils to synthetic. Both diff, engine, t-case. Get smoother rolling and lighter weight tires. Michelin green X are good MPG gainers.
THere was a mention about linking trips. Cold engines get single digit mpgs. So if all of your trips are 5 miles or less, you won't ever get great mpg.
And really from my experience the type of roads (speed,levelness, and # of stops) contribute the most to the type of MPG. With summer gas and my camper shell off I can get close to 20 mpg. If I only did my commute (which has the most ideal road conditions, 45-55 mph, downhill start when engine is cold, 20 miles each way, level, 5 stop lights) I definitely can get over 20 mpg.