Is it my imagination, or does my car run better with a full gas tank as opposed to one only [sup]1[/sup]/[sub]2[/sub] full?
I think that, when I pull out of the station after a fill-up, I notice that my car accelerates faster and more smoothly than it does, say, three days later.
Does this make any sense?
The level of gas in your tank makes no difference, unless it’s so low the fuel pump is sucking in condensation, crud, etc. Psychologically, it could just be that after you clean the windshield, check the oil, etc. you feel better about your car.
However, a full tank is heavier (obviously) than one near empty. Could you be getting a hair better traction, sitting lower on the road, etc., that makes the car feel more sure? Try throwing a 50-lb. bag of sand over your rear wheels when the tank is near empty, and see if the feeling isn’t the same.
There are two more advantages to keep your tank as full as possible:
If you notice a difference in acceleration, you may be getting better mileage on the top half of the tank too. There are a couple of reasons for this. You’ve probably got a bit of crud and water in your tank. If there’s only a couple of gallons left, there’s more of it mixed in your gas, which means it’s getting in your engine and gunking things up as well as affecting performance. This is probably the best reason not to drive an older car all the way down to empty.
The other thing is that engine performance is determined not only by the quality of gas but in how much air is mixed in the gas. If your tank is lower, there’s more air in there to slosh around and get mixed in the gas, before it gets watered (aired?) down even more by your carburetor (if you have one).
In my experience, you’ll notice the difference more in older cars. My 16 year old Volvo generally seems happier, and on average I get about 1 more mpg when the tank is full, but my '68 Olds got downright cranky if I drained the tank. The difference was more like 2 or 3 mpg between full and half empty.