1996 Buick Century temperature gauge not working properly

Ive replaced the thermostat and the sending unit. So ive probably got a bad gauge. The thing is though I get poor gas mileage like it never warms up. The gauge only goes up to the 1/8 mark and it takes awhile to get there. I drove 150 miles highway at 55mph and grandma take offs n only got 20 mpg. Did I get a bad thermostat or is something else wrong. No check engine light though