2000 Accord, 4-banger

With gas nearing the 4 per/gal. mark, I have been paying alot closer attention to how many miles I can go with every milk jug of liquid gold that I pour into the gas tank. Of late, only 19 mpg(average over 5 refills); but before, this car could get 30 mpg with soft foot on the petal. The two major repairs done to the car were putting in an after market catalytic coverter after an engine light come on and replacing the two half axles with refurbished ones because of a badly torn CV boot on one of the axles.

Someone said it might be the cheap cat. coverter(original cost well over six bills) I put in or the BRAKES ARE DRAGGING. Is there a way to troubleshoot if one of these items might be the cause and what does dragging brakes mean?

Before taking the car to the mech. I wanna gets some ideas as to what might be wrong from some very knowledgeable people here. Thanks.