Sudden drop on MPG causes?

That is why I continuously monitor my air fuel ratio with my ScanGauge. If something was changing in my fuel delivery system, it would change my air fuel ratio from the normal of 14.7(for a gas engine) at cruise to some reading that would indicate my engine was burning more fuel than normal.
It takes a period of time to accurately monitor fuel mileage but my air fuel ratio is displayed instantaneously so if a problem develops I will know it right away.

More vote is for one or more bad coils.

Mr.Mark wrote:
This is a good example for checking fuel mileage. I have done it at every tank on our coach (even though it's diesel). If a person didn't check their mileage do you think this would have been caught?

MM.

X-2, I check my mileage like Mark at a minimum of at least once a year! Very good practice!

Thought it was summer blend that dropped due to higher vapor temp.. Oh well, what do I know.

I know they used to tell us Gas and Booze do not mix, but now the government insists we buy a mixture.. Those "Do not mix" folks knew their stuff (Save for a small amount in the winter)
E01 Good, E02 Good (in winter) e more than 5, not so good.

Winter blend fuel will drop. Definetly get an analyzer on it.

Are you sure it's 3 MPG? Short trips can kill mileage.

You do mention an 800 mile trip though. Did you really have to stop every 150 or 200 miles (50 - 70 gallons) and fill up? If you were running that rich (double) you'd be pouring black smoke out the exhaust pipe.

(And the whine is the cooling fan, I'd bet money on that.)

I appreciate all the ideas... Regarding stealing my gas, not likely, I have measured the MPG from fill-up to fill-up while on the road, not after storage.

I am not sure if it is related but sometimes when pulling really hard on an incline I hear a high pitched whine that continues until I let up on the throttle.

Thanks again for the help.

Defective O2 sensors can cause this to happen, but should set check engine light. Have seen some that did not however.

Is it possible someone is taking 5 gallons out of it when in storage?

If it's a Ford (you didn't say) a plug/coil could be misfiring or causing a dead cylinder. The engine is smart enough to compensate that you wouldn't even feel it missing but your mpg sure will suffer.