new to me 07 questions

[Jerry Redtruck]

Hi, I used to be on this site way back when I bought my new 02 dually. Been through injectors and all the issues.
I decided I didnt need a dually anymore and found this 07 chevy crew cab short bed. First thing the fuel gauge was all the way down to E and the light was on so I headed to fill up and I could only squeeze in 21 gallons. Is this right? does this truck have a smaller tank? Could my gauge be bad? On my dually I could fit 28 to 30 gallons with the needle still above the empty line.
I also noticed after I had bought it that the wheels are 17 inch GMC. The sticker says stock should have been 16. I guess they are off a newer truck. Will this be a problem with running too big of tire? I need to change the tires to some aggressive all terrain type and may as well change the rims while I am at it if it is a problem.
How do I know if I got the locking rear end?
Previous owner claims 23 mpg. I dont really believe that after never being able to do better than 13.5 with the 02. But I am optimistic I will at least do better than the 13.5
I tried scanning through the manuals that came with it but couldnt find the answers, thanks if you are able to help me out. Jerry