I have an '89 F-150 Custom that has a definite problem with guzzling gas, But it doesn't do it all the time. I've found that, in general, gas with a higher octane rating gets better mileage and also makes my truck run much, much smoother.
One day I put 10.5 gallons of 93 octane in, and I set my trip meter. I got 14 mi/gal. out of that 10 .5 gal. A couple of days later, I put in 10.5 gallons of 93 octane in, just exactly the same as before. I reset my trip meter. I went the speed limit, just as before, and took the same exact routes home, and to work. This included 1 trip on the Interstate highway, traveling at the same speeds. This time, I got roughly 9 miles to the gallon. This happens intermittently, and i can't for the life of me figure out why.
My check engine light was blinking on and off, so I took my truck to a local auto parts store and had them hook it up to a computer, to diagnose the problem. The results were: EVP sensor below threshold, Vehicle Speed Sensor: Insufficient Input, and Fuel Pump Primary Circuit malfunction. I took the EVP sensor off of my truck, which has 212,500 miles on it, and put the EVP sensor from my parts truck, which has 162,400 miles on it back on my truck. The check engine light stopped blinking for a short while, then slowly but surely began blinking on and off again. Please Help!!

Thanks,
Lee