Fuel gauge and mileage issues...Help

I have a 2010 Rocket 3 w/ 5k on it. I had the triumph slip on pipes installed along with the triumph map for the pipes. The sound is richer and there is a bit more rumble in the seat. The issue I'm having is when I fill the tank, The gauge will reset it self to about 165 miles to E, and then clime to around 207 til E, then at about 25 miles on the tank I can see the fuel gauge drop to the half mark. I know this is a false reading ...cause when I remove the filler cap I can still see the fuel level. I have also noticed my mileage has been reduced, before the slip on pipes I was averaging 36 to 42 MPG depending on how hard I pushed. Now I'm looking at 18 to 25 MPG. The slip on did not change the HP output, so would it have that effect on MPG?

It's been the general consensus here that the R3 fuel gauge is as worthless as tits on a boar hog. Don't worry, you aren't alone ... it's just one of the few little things we overlook to ride a Rocket. Most rely on the trip odometer and experience with the bike, the only real way to figure mileage is to average quantity of fuel used and miles ridden over a few tanks.

Mileage can vary so greatly from just a twist of the throttle when you're talking about a 2300cc motor, 1,000lbs and grinning from ear to ear.

thanks for the input...planing a 1200 mile ride in two weeks, and just trying to reduce the bull**** list.