Pepino, something is waaaay wrong - your mileage should not be that bad no matter how fast you drive. I drive ~90 mph on the highway to VT and I get at least 13 mpg.

If you have an oiled air filter (like a K&N) then you should clean replace your Mass Air Flow sensor (MAF or MAS). It measures the air coming into the engine, which the ECU uses to decide how much gas to squirt into the cylinders. The FPR regulates the pressure of the fuel in the lines, which also affects how much gas goes in. If either one of those is off, so will your fuel mixture.

You can get a new or rebuilt MAF at AutoZone, or you can use CRC MAF cleaner.