I have a 96 XJ, straight 6, 2 inch lift in the front 3 in the back, 31" tires, magnaflow muffler. With that I was getting decent gas milage, around 12 or 13 miles to the gallon. A couple weekends ago I put a spectre cold air intake on (suppose to increase horsepower and mpg). I noticed it seemed liked I was running out quicker so I filled it up, tracked the milage for a week and filled it up again, then divided the milage by the number of gallons I used. Now I'm sitting at 8 miles to the gallon. Any help? Thanks in advance.

Oh and right before I put the intake on, we changed the oil and did a "tune up"

Did you replace your O2 sensors ????

No I didn't, is that something that would cause the check engine light to come on?