I have been having problems with my truck getting really bad gas mileage. It has 194K miles on it, and was running pretty well with the stock gears and 31x10.5x15's. In November I got new tires in 31x12.5x15. The truck is sluggish and weaker than normal, seems to run rich,and the exhaust smells strongly of gas. I used to be able to get 300 miles per tank (3.0 V6) and I am down to around 200 miles per tank. I suspect that it is the O2 sensor since it is supposed to be replaced every 100K miles and I know that I never replaced it. Any ideas? I recently replaced the cap/rotor/wires and plugs and it runs the same. I bought the O2 sensor and an Equus code reader but I can't do anything since the rig is at the body shop since I was in a wreck in Oregon in December. The truck can still do 70mph on the freeway but only if it is floored and up to 4th gear. Any ideas?