1993 Civic Si Hatchback getting bad MPG

[92VXHatchguy]

Hi, I've recently began to notice that my 93 Si Hatchback has been getting worse in terms of fueling in between tanks. I remember getting around 300 to 350 miles from a full tank, now I'm getting around 200-250. I don't drive the car hard, but it has approx 247600 miles. I just did an oil change, cleaned out the IACV, replaced my cat converter with a fairly newer cat (it wasn't passing smog with the original cat). What else can I do to get my higher mpg's back? I've owned the car for 6 years and plan on keeping it for many years to come. I've read a bad o2 sensor could lead to reduced mpg's, I don't believe I've changed it since I've owned it. Any advice would be appreciated.

[theholycow]

I imagine that an O2 sensor from that era can suffer from being "lazy" without throwing a code on an ECU of that era. That might indeed be a good idea to check.

Of course you should have a fuel log to give you not only a better idea of what's going on right now but a better idea of seasonal patterns too.

By any chance does the new catalytic converter correlate with your loss of fuel economy? Perhaps the new one is messing with efficiency even if it's not restricting flow, or perhaps it is defective or it failed shortly after being installed.

[92VXHatchguy]

I'll look into getting the o2 Sensor replaced. As far as the catalytic converter goes, I was getting the same mpg's. I thought after I replaced it with this newer one, my mileage would have gone up. My car first failed smog 2 years ago, was considered a gross polluter. I changed the cat and still noticed that it was getting bad mpg's.