2001 Ford escape 3.0L V6 Gas Mileage Problems

I recently bought a 2001 Ford escape that has gas mileage issues. at first i was getting decent mileage at about 15ish then it got worse into the 12-13 range, i changed out the oil to a full synthetic and replaced all the spark plugs and boosted me to a remarkable 20mpg for about a few weeks then dropped back down to around the 13-14 range again. I looked around the engine and saw some brittle vacuum lines and replaced them. this fix did little to no change at all. Also the car had a brand new fuel pump, fuel filter and K&N air and oil filter. I have been stumped for several months now trying to get the gas mileage it should be getting. As a college student money is scarce and would like to be able to get this solution solved so i can save money at the pump.

Thanks
Mario.

How is the acceleration? Can you get up to speed with the tach staying at or below 2,500 rpms, or does it have to jump up to over 3,000? Does it down shift on the highway for medium sized overpasses? If so, the catalytic converter could be clogged. Some engineer had the bright idea to put small catalytic converters as part of the exhaust manifolds. They have a honeycombed ceramic interior that falls apart. Then those guts travel back to the one large main cat and clog it up. Every couple of months I take my main cat off and bang the inlet on the concrete and tap the tube with a hammer to get it all to come out until I can see daylight through the internal mesh again. I usually get about a cup full of debris.

The acceleration does seem a little rough, for a slow acceleration to 25-30mph it goes to an average of 2500rpm fast acceleration launches up to 5500rpm. At highway speed I have to force it past about 3500rpm to get over 60mph, then it drops down to about 2000rpm.

My bet is clogged exhaust. If it is bad enough, the EGR valve will explode from the back pressure.

In you opinion should I just do a straight pipe exhuast or just clean out/replace the cat

If I didn't have to be emission tested every other year I would have gone straight pipe. That is what I would have done. My advice that doesn't break any laws is to try and clean out the cat or replace it.

Are there any sensors that I need to be worried about if I perhaps did a straight pipe since emission test are not required in my town.

The sensors are in the system before the main cat and should not be an issue.