I have been watching my 91 Civic's mileage particularly
closely since about October. This includes, for the
overenthused, watching the fuel tank gage. A few times I
have thought to myself, "Darn, it's reading just about
half-full, and I usually have at least X miles by this
point. The trip odometer is at more like X-50 miles right
now." So I would predict that the next fillup will yield
stats indicating really bad mileage. But on the contrary,
apart from a few weeks where I had the timing messed up, it
looks good, for winter.
As people have indicated here recently, gas pumped in the
summer from a nice cool underground tank (typically) expands
once in the car's tank and while warming to ambient. This
makes sense. In this vein, could it be that, while driving
in the summer, the fuel tank gage reads particularly
disproportionately to the lbs. of fuel consumed? That is,
the actual level in the fuel tank goes down literally more
slowly from full tank to half, because the gasoline in the
tank is simultaneously expanding (due to temperature
increases). By the time the driver reaches a half tank or
so, the gasoline isn't expanding as quickly, because its
temperature is pretty constant.
I recall times in the summer where my Civic has achieved
nearly 300 miles by the time the gage reads half full. Then
it drops very quickly. In winter, I can't get anywhere near
as many miles on the trip odometer by the time the tank is
half full. Still great mileage; just totally out of whack
with the fuel gage.
Anyone else notice this? Comments on this theory?