First off, how much is your speedometer wrong? Never discount that as being a MAJOR source of MPG miscalculations. Tire size is almost never as advertised, has the tranny ever been changed, have the gears been changed, etc.
If this was done with GPS ignore the above /forums/images/graemlins/smile.gif However, it can't be overemphasized that mileage HAS to be measured correctly for MPG to be computed accurately.
Two of the most important things to check are plugs and wires. Short of that, on a stock setup, completely worn out engine or carb problems are going to be the only things that will make a "huge" difference in MPG in my experience.
I fixed my distributor with sticky mechanical advance, rebuilt the carb, made sure all the plugs were clean, gapped them differently for testing, and so on, but nothing made a huge difference in MPG (and driveability) like changing out bad plug wires. You expect something like mech. advance to really wake up the truck when it's "bad", but it just didn't make enough difference to notice.
Of course, I'm getting 18MPG freeway in optimal circumstances, but very little of that is some trick I've performed. (and yes, that MPG was BEFORE I fixed the advance weights, didn't change afterwards) The only real thing I spent any time on was the primary rods/jets in the carb, and only messed with those because the carb wasn't set up for this engine, and was surging at cruise. Not trying to start a "my truck gets XX MPG" thread, because it's been done to death, but just letting you know whats possible, and people that are getting much worse than that on a fairly stock setup, well, somethings not right.
