Fuel?

So, I've spent the past hour searching and mulling through the site trying to get a solid answer, but have come up a bit short.

I just bought a 2011 Colorado 2.9l, 4sp auto ext cab on stock tires. Only mods are k&n cai and some weird sounding magnaflow exhaust. I have a bed cover also. The truck is in mint condition, runs perfectly and has been well maintained as far as I can tell. 68,500 miles.

The issue im having is with fuel mileage. Im thinking its a bit low. I've gone 150 miles and am right at half a tank. I've run the ac/defrost a bit and the transmission gearing on this thing is a bit wonky, so sometimes you have to get on it to make it downshift in order to maintain 70mph. Given this, does 150 miles on half a tank sound a bit low to you guys?

What are you with the 2.9 seeing at half a tank?

I know from my search, mpg is a hot topic and I'm not trying to beat a dead horse. I'm just a bit concerned as I have a 30 day, 100 percent warranty, and 3yr/36000 mile warranty after that, and if something's wrong, I wanna get it looked at by one of the techs at work (I work for the dealership where I bought it)

Thanks!