92 civic vx 29mpg ??help?

[amaterasu71]

Hi everyone, I bought a 93 civic hatch dx for my kids this fall it had 60k original and it has the 4 sp auto. I was so impressed with it as it got 43mpg on the freeway that I kept my eyes open for my dream mpg car the VX model. Just bought this vx here is the stats.. 199,000 miles, new tires, new exhaust, im the 3rd owner, and the car appears to be in great shape as there is just the start of bubbleing of rust spots. the engine light is on and the previous owner told me it was the o2 sensoer, he siad it was really exspensive to replace so he never did. So on my initial trip it only pulled 29 mpg. very dissapointed so im going to change everything oil, filter, air filter and such my question is would the o2 sensor be the cause of such low mpg? what other things should i be looking at? I was hoping for atleast 40 on the open rode i only was driving 55 mph and im really easy on it. looking forward to any help thanks, shawn