Help! I'm only getting 320 miles a tank!

[hitmoninfinity]

I recently got new tires and wheels. The drop in mpg seems to have started around the same time. I'm not sure if that's the cause though. Both the tires and wheels are the same size as before. Does anyone know what the stock steelies weigh? My new rims weigh 17lbs each.

But now that I think about it, I think I was only getting 330-340 a tank before. I keep seeing all you guys in the 40 mpg range! I'm not a diehard mpg junkie, but I do drive pretty reasonably. Enough to be appalled when driving with anyone else. What can I do to get up towards 40 mpg?

Has anyone had to replace an O2 sensor yet? -Did it make a difference?
Is there any other maintenance issues I might be overlooking?

It's a 07 sedan with 50k on it.