92 Bronco O2 sensor and EGR valve

Hey guys,
I have a good 'ol 94 Bronco with a 302, just purchased it for the winter months. I bought it just before it started to get cold (October), and never had the chance to check the mileage under optimal conditions. Now that winter is here I am warming it up, making short city trips, using four wheel drive (sparingly), and other usual winter driving practices. Anyway, I'm starting to think my Bronco is excessively consuming fuel, or at least more than it should be. I put 5 gallons in the tank and it seems like I burned it up within 2 days of city driving (less than 20 miles). When I noticed this, I filled up the tank, took a 50 mile road trip (all highway), and filled it up again and only got 11.2 mpg. The previous owner said it got 15 (I presume during the summer, and he might have exaggerated ) Are there any Bronco or F-150 owners who get this kind of mileage in the winter?

Also, shortly after I bought it the check engine light came on (of course, right?) and during warm up it would idle rough. I borrowed an OBD I scanner, used it, and got following codes:
173 Lack of heated O2 sensor switches indicates rich bank 1
327 EGR valve position or exhaust gas pressures transducer circiut below minimum voltage
332 EGR valve flow insufficient
452 insufficient input from vehicle speed sensor to power controle module

I got a few other codes (628, 636), but I think the cold weather had something to do with those.

After doing some research, I realized that the EGR valve is responsible for the rough idle. I plan on cleaning my EGR valve this weekend, and possibly replacing the O2 sensor. But my question is: would the failure of these 2 systems really effect my mpg to that extent? Is a rich running vehicle really detrimental to mileage? I wish I would have purchased the vehicle earlier so I could have gotten a better understanding of it before winter set in. 11 mpg on the highway the seems quite low, even for relatively modern full sized vehicle in the winter. Thanks for answering my questions.

Brandon

if you havnt replaced the o2 sensor already i definatly would that should dramatically increase your fuel mileage...when vehicle is runing rich due to faulty o2 sensor it will use way more fuel then needed also when you have time check the spark plugs as faulty o2 will cause them to wear out faster then normal if bad replce them along with wires and cap and rotor and will boost mpg even further good luck...