Gas milage vs half full tank

[hjwii]

Let's see... I've got a 2005 Vibe, 5spd with 57500 miles on it. I average about 34 miles to the gallon, and usually put about 10 gallons of gas each fill up. Lately, I've been filling up when the needle is between 1/2 and 1/4 full (this is due to the fact that gas is almost 20 cents cheaper in New Orleans than the outlying areas, and the way the commute falls, I would have to fill up outside the city...anyways.....) So, I'm putting about 7 gallons in each time. My gas milage is now hovering around 40/41... tell me, why is this? Now, I'm still calculating my milage the same way, so I haven't changed the equation...So... someone out there in Vibe land... tell me if I am getting better gas milage, or is this some sort of trickery by the oil companies to ensure that I visit their stations more and more whilst they rape me of my boy's college tuition funds...hjwii

[Mavrik]

no idea but the top half of the fuel tank seems bigger then the bottom half IMO as my needle drops faster after half a tank then to that point...

[Baltovibe]

http://forums.genvibe.com/zerothread?id=18687

[joatmon]

You may already be doing this, but fill it up each time, and add up several tanks to get an average. So, if youstart with the tank full, then drive the 280 miles, and fill it up and it takes 7 gallons, then drive the next 280 miles, fill it up and it takes 7 gallons, etc after say five cycles you'd have driven 1400 miles and it only took 35 galloins to fill it afterwards, then I'd say you are getting 40 mpg. If one single tank time it was 7 gallons after driving the 280 miles, then it might be the normal variance in how full is full for that particular fillup.

[jake75]

Also - NUMMI needs to take a look at your engine and see how you are getting that many miles to the gallon.

[ZubenElGenubi]

Just throwing out a couple of contrary ideas.First, gasoline weighs about 6.25 lbs per gallon (or for you metric types: about .75 kg/litre). The more weight the car has to carry, the lower the mileage. One should expect, therefore, for a car's mileage to be less when the tank is near capacity than with it nearly empty. The effect is probably very small considering the weight of gasoline in a full Vibe tank is only about 84 pounds (38 kg).A possible countereffect might be loss of fuel due to evaporation. Gas tank caps should prevent that, however. Also, the amount of fuel loss required to explain your results would be about 1.5 gallons!7 gal * 40 mpg = 280 miles (partial tank fillup)10 gal * 34 mpg = 340 miles (full tank fillup)Difference:3 gal * 20 mpg = 60 milesat 40 mpg, this would only be 1.5 gallons, therefore, you'd be short 1.5 gallons by the time you fill up another 10.This seems highly unlikely.A leak is also unlikely because no matter where the leak could be in your tank, the loss would decrease as fuel was emptied, resulting in better mileage over the course of a tankful.

[Gandalf]

I'd *love* to get 34mpg. I have a base AT Vibe, and the best I can do is 25 or 26 mpg.