1997 FORD F-250 HD 460 7.5L 350,000 MILES

USUALLY GET 8-10 MPG, DON'T KNOW WHY IT HAS CHANGED. I HAD THE HEADS R&R FOR A VALVE JOB & RESURFACED DO TO A BLOWN HEAD GASKET IN MAY 2009. SINCE THEN I HAVE HAD POOR MILEAGE AND SOME MINOR TIMING ISSUES. I HAD TO REPLACE THE TRIGGER WHEEL WHEN IT HIT THE MISFIRE SENSOR, SENSOR HAS JUST A NICK IN THE PLASTIC ON THE SIDE. HOW DO I FIND THE SOLUTION TO THIS PROBLEM?

If you're talking about the crank sensor, that is more than just a misfire sensor. The comp basis a lot of it's calculation off it's input.

The 1st thing would be to know if your check engine light is on and if it is what is the code. If not the next thing I would want to look at would be the data stream and the short & long term fuel trims to see if they are negative, also take a peek at the O2 crosscounts since you mentioned you blew a head gasket and then go from there depending on what turned up.......