Quick question on gas milage

[trevorjk]

so my car over the last 6 months has been averaging about 30-31mpg under normal driving conditions. over the 6 months i would drive until the light came on which meant i had about 1.5 gallons left in my tank.

now the last 2 times i filled up, i filled up with a quarter tank left in my tank. and both times i have done that i have gotten 33.5 and 34mpg while still driving the same as i normally do with no odd trips on the highway. (daily normal driving)

my question is this. why is this?

i have always thought that with less weight in the car (less gas) you should be able to get better mileage, but it seems that when i fill up about 60 miles before i use to i get 2-3MPG higher then i would normally get

[y0da900]

Not entirely sure, but I would imagine because you are allowing less of an opportunity for the liquid to turn into vapor by occupying it's expansion room. Your fuel pump pumps liquid, not fumes.

[thecavemankevin]

Not entirely sure, but I would imagine because you are allowing less of an opportunity for the liquid to turn into vapor by occupying it's expansion room. Your fuel pump pumps liquid, not fumes.

i think he is right. This sort of theory is also why you should fill up in the early morning or late evening when the day is coolest. The gas itself if more dense, thus you are getting more for your money.

[Lohman446]

The liquid around your fuel pump allows the fuel pump to dissipiatt (sp) heat more efficiently. While I doubt this has anything to do with mileage constantly running your vehicle low on fuel (below a quarter tank) will shorten the life of your fuel pump.

[Crighton]

E85? did you fill with the same gas each time? 10-15% ethanol?

[rkjunior303]

:sigh:

30-35...

I wish.

the 20-25 i get on my wrx with 93 octane is all i can muster.

[93civiccpe]

Lohman is correct. A friend of mine who is a mechanic told my dad that the gasoline actually helps keep the fuel pump cool and that by driving until you are almost empty it causes that fuel pump to heat up and reduce its life. I'm not sure if that in any way affects your gas mileage but I know my dad fills up more often after he learned that when his fuel pump died.

[trevorjk]

E85? did you fill with the same gas each time? 10-15% ethanol?

i use 89 or 91 octane with NO ethenol in the gas. (we have a gas station that sells mid grade for the price of cheap gas on tuesdays )

and thanks for the reply guys, i think i will be filling up afterwork when i have a quarter tank left .

[Blazestorm]

If you car is supposed to run 91, give it 91, or if it's supposed to run 87-89... give it that...

"Premium" gas isn't higher quality, it's higher octane.

[regal1975]

the gas itself makes a huge difference.. my car gets from 27-47mpg depending on the fuel... 47mpg with 93 octane from tenessee (no additives, no smog crap, no ethanol) and 27mpg with 91 octane from Iowa (ethanol crap)..

[Blazestorm]

Sounds like your calculating it wrong...

Most gasoline cars can't get 45+ mpg... and if they do, they wouldn't drop down to 27mpg... sounds too ridiculous :P

[kevdupuis]

:sigh:

30-35...

I wish.

the 20-25 i get on my wrx with 93 octane is all i can muster.

Wow I get 30/31 on the highway with my Magnum on regular, of course that's with the cruise set at 70 mph.

[Lohman446]

If you car is supposed to run 91, give it 91, or if it's supposed to run 87-89... give it that...

"Premium" gas isn't higher quality, it's higher octane.

Theres a point to this. Unless your engine is built with the compression for it (or you are constantly using a turbo / supercharger) higher octane fuel will not help you at all.

Higher octanes fuel burns slower and longer than lower octane fuel. Because it burns longer it creates more heat in the engine compartment. Octane also influences how much a fuel may be compressed before it spontaneously ignites.

Modern vehicles are designed around a specific octane rate. GM for instance specifically advises not using mid or high grade fuels in vehicles calling for 87 octane.

The use of a higher octane fuel increases heat in the engine compartment (though this is probably minor). GM argued it may cause ignition issues, that without the compression high enough to be close to igniting it the plug may not burn it fully, and running a higher octane fuel in a vehicle not designed for it, if it has any effect at all on mileage, will actually decrease your mileage.

No good can come of running higher octane fuel than your vehicle is designed for in normal driving (theres no argument there from anyone with knowledge). It may help in extreme RPM applications (there is some argument there). That being said, in your everyday driving application the only thing that higher than called for octane fuel can do is harm your engine and lower your mileage.

Vehicles designed for higher octane fuel (through compression, turbo, or supercharging) will notice a performance drop off from running lower octane fuel (as the knock sensor adjusts timing). They are not going to suffer any drawbacks from running the fuel they were designed to run - running 87 Octane in your supercharged 3.8L (high compression custom engine, turbocharged whatever, etc.) will not get you better mileage, you will lose performance.

If your engine is exhibiting engine "ping" or preignition it is exactly what it sounds like. The fuel is igniting from compression rather than spark. In these cases running a higher octane fuel may get rid of that condition and be beneficial to you. However, this is a malfunction and should not be considered in the "everyday" driving category.

[MANN]

the gas itself makes a huge difference.. my car gets from 27-47mpg depending on the fuel... 47mpg with 93 octane from tenessee (no additives, no smog crap, no ethanol) and 27mpg with 91 octane from Iowa (ethanol crap)..

Im in TN, and we have 10% ethanol at all of our pumps.

[trevorjk]

Im in TN, and we have 10% ethanol at all of our pumps.

im in wisconsin, and about only HALF the counties have ethanol. but if you drive 10 miles to a neighboring county chances are you can get gas with out ethanol. luckily for me, i live in walworth county that does not require ethanol in the gas.