check the rotors / drums for warpage, and check the wheel bearings. Jack the car up, put it in neutral (key off) and manually rotate the wheels one by one. If you hear a "dragging" or light grinding noise only at certain locations throughout rotating the wheel, then there's your likely culprit. Either one will make your gas mileage go down, and cause a grinding or thumping noise when driving.
Even if your tires are balanced correctly, it will still feel like something is out-of-round.
Also, check the amount of "play" in the wheels by grabbing the tires at positions 9 o'clock and 3 o'clock and jiggle the wheel. If you have an unreasonable amount of movement (light clunking), then higher likelyhood to the wheel bearings being bad or center nut needing to be retightened (check required torque values).
Thirdly, check to make sure all your inner CV's are tight.
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