I have a 97 honda civic i bought it with no cat just straight piped its a 1.6 non v-tec 4cyl?

I feel like i get bad gas mileage so if i put one back on should i get better

YES. I would recommend that you install a cat/ o2 sensor if it doesn't already and a muffler and you will see way better mpg's. Only go stock size pipe or one size bigger than that. Many honda people think if they put on a 3" exhaust or straight pipe it like your case, itll be quicker but that is % 100 wrong. 4 cyl motors need some back pressure to get the most out of the exhaust system, nox, and what it was intended for.