Better mpg after filling up?

[sheepdog 44]

This is just anecdotal evidence, but for the 3 cars i've owned, it seems that after filling up i instantly get better mpg and power. This effect goes away the next day after the car has settled. It's "feels" like enough of an improvement that i'm probably not imagining it. In my previous cars i noticed better acceleration. With an instant mpg gauge, i can see i'm getting the mpg i would maintaining speed, but while i'm slowly accelerating.

Has anyone else noticed this?

Could it be vapor in the gas tank? Fresh fuel? New fuel in a warm engine?

[Flakbadger]

I feel like my vehicles have always responded to new gas like you said. I wonder if it's oxygenation, or just in my head...?

[pete c]

It's in your head. Gas has weight. Weight slows your car. End of story.

A full tank may improve handling by improving weight distribution, but that is about it.

[gone-ot]

Mathematical Delusion - The MPG variations are due to the "Law of Large Numbers."

Initially, right after filling up, the MILES and GALS numbers are both small and little changes are immediately seen on their result, ie: MPG = MILES / GALS

...for example, small initial changes:

MPG = (10/0.3) = 33.3 mpg ...BASELINE, 10 miles & 0.3 gallons.

MPG = (10/0.2) = 50.0 mpg ...-0.1 gal gas
MPG = (10/0.4) = 25.0 mpg ...+0.1 gal gas

MPG = ( 9/0.3) = 30.0 mpg ...-1 mile
MPG = (11/0.3) = 36.7 mpg ...+1 mile

...which are variations of +50%/-25% for gas and ±10% for mileage.

However, as you accumulate mileage, the MILES number becomes large enough to 'swamp' similar changes in the GAL number.

...for example, here are the same changes as above, except at a distance of 150 miles (same ×15 ratio):

MPG = (150/4.5) = 33.3 mpg ...BASELINE, 150 miles & 4.5 gallons.

MPG = (150/4.4) = 34.1 mpg ...-0.1 gal gas
MPG = (150/4.6) = 32.6 mpg ...+0.1 gal gas

MPG = (149/4.5) = 33.1 mpg ...-1 mile
MPG = (151/4.5) = 33.6 mpg ...+1 mile

...which are variations of just ±2% for gas and ±1% for mileage.

Thus, as the mileage and gas input numbers get larger the the resulting variations in the MPG numbers become smaller, even though the changes were identical!

[Cobb]

Yall wrong Its the fact you get over spill in the carbon filter and it enriches the air charge into the engine til it evaporates off for a few hundred miles tops.

[sheepdog 44]

I'm talking instant mpg and power just after putting gas in. Which lasts till i park the car and it sits overnight. Normally i do ten miles after a fill before i get home, so i don't know how long it could last. I'm not saying you could improve your mpg for a full tank. But there is definitely something going on with how an engine runs just after putting some gas in.

[BHarvey]

The drive home from filling up is done with a warm engine, then the next day you suffer your first cold start on a new tank and MPG suffers.....

[pete c]

So, what you're saying is the gas gets stale?

[Cobb]

Its because the filter on the hose contains magnets and they help to align the atoms inthe fuel as it enters your tank. Then the charge wears off the next day and your performance is less?

[wmjinman]

I seem to notice the opposite effect - I'll be getting a particular MPG for a tank, and then fill up, all excited to improve upon it (warm engine, etc.), especially on a trip. But the ScanGauge will show much LOWER mileage for the first few miles. Seems like I can get it to start climbing eventually, but not at first.

I do try to top off the tank until it won't hold any more, thinking that's the only way I can be sure it's the same each time. Is this somehow wasting gas (burning extra out of the carben cannister & running rich, or something).?

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?