Car is only getting fifteen mpg when it should be getting thirty three mpg

I cannot figure out the problem. I looked on the fuel economy website and this car is supposed to get thirty three city. That is why I bought it. But every time I fill up, it calculates out to about fifteen mpg every time. What could cause this bad of mpg? It is not reading any codes either. I have tried injector cleaner but as I thought, it made no difference. Please help.