1998 Nissan Sentra fuel consumption problem

If it were me I wouldn't mess with the throttle body especially if its idling fine. Do you have any check engine lights? If not, try putting some fuel additives in there. The stuff that I use is some lucas fuel injector cleaner which costs about$5-8 at autozone or advance. Put new plugs, wires, cap and rotor on it or just do the plugs. Your alignment and a bad caliper can kill your mileage too. Also check your tire pressures too which should be around 32-35 psi.

Or you can also put on a cold air intake which increases 8-10 hp and it does improve your gas mileage if you drive it conservatively. If you keep launching it at high rpms with it you would burn more gas as if you were to have the stock intake on there. I know from experience and I used to do that since I loved the sound and my mileage went down but when I drove it regularly, my mileage is awesome. I don't know how your driving style but if its not good slow it down a little lol.

If you have anymore questions feel free to ask. Hope this helps. That's all I can think of for now.