Your location provides a bit of a clue:The weather up there means that you would be more susceptible to water infiltrating your fuel supply. I dealt with this during the winter when I lived in Chicago.

What'll happen is that when it gets cold, any moisture that's in the air in the tank will create frost along the sides of the tank. As it warms, it will condense into liquid form and drip down to combine with the gas. As you run the tank down, there's more air in the tank, which becomes more frost, then more condensation, and more water in the fuel.

Water in the fuel produces less energy, which will translate into poor mileage.

Keeping more fuel in the tank makes for less air space, less water vapor, less water in the fuel.

 

That is the first logical explanation I have seen. Is it enough to make that large of a difference (1-2 mpg)? The same thing also happens in the summer, but it does cool down at night (is this a big enough temp change?

Matt

 

I noticed a long time ago that I get a lot better gas milage with Shell gasoline. I always get at the very least 10 mile more per tank with Shell.

Just my $.02

 

Hmmm... Frost comes from when temps approach freezing, but you can have condensation in much warmer temps (think about the mositure on the outside of a chilled glass as it warms). The issue at hand is the temperature _difference_, not so much the temp itself. So, say you had 80* (Fahrenheit of course ) during the day, and 40* at night, that's quite a swing. There also may be a difference in the fuel mixtures you see there - more/less ethanol will affect the ability of the fuel to absorb moisture.

The short form is that a swing of 1-2mpg is definitely possible from increased water levels in the fuel. Sounds like you'd be wise to keep the tank more full than not.

 

Well as I have said above, I never stop at the shutoff, I always let it go a bit more. And as you have seen with my professional () calculation above, putting MORE into the tank will give me LESS MPG in the calculation. I would have to put in 1 gallon less than I usually do to get that result and I never do that. I always fill it to the top and then round of the numbers. If I didn't do that, all my mpg's would be in the 15's and the shell one in the 17's.

tc...I will fuel up with Shell the next few times and report results. I am not gonna be the guinea pig for all other gas stations. Shell is a big one and Chevron is too and I would be amazed if it actually turns out Shell gives you better mpg.

If it indeed has to do with the gasoline, I wonder what chevron is doing to theirs? More additives, less gasoline (cheaper)...dunno. We shall see.

 

Something I do know is % of ethenol in the gas will affect your mileage. The ethenol is mixed in at the distribution center and different companies may add different amounts.

 

By any chance did you disconnect the battery or try to reset the computer after you put the O2 in?

James

 

I always wanted to say this...BUSTED for now...

After I discussed this issue with all the people in this thread, I decided that I would try to fill up my truck with Shell gasoline (87 octane) on my next tank. I didn't do a scientific method approach because I wanted to leave that for later in case I get another 16MPG tank.

Well guess what...on my 1st experimental Shell fill-up, I get 14.5 MPG, the regular-ol. So, I decide to go Shell 87 again and I get 12.2MPG!!! HELLO? Note: I always stopped filling at the first click for more accurate results.

So I really don't know what to think. My highest and lowest MPG were both achieved with Shell at the same gas station. The only possible explanation I have is the gas station. They used to use USA gasoline, but now they use Shell gasoline with a USA shop mart, if that means anything.

Obviously, I am being forced to do more trials on this, so coming up, I will setup an approach on how to test these two top-tier gas stations fairly and efficiently with a low margin of error. Any ideas/help will be appreciated.

One Thing:
I will only test Chevron and Shell. Those are the two best gasoline stations (additive wise) and are both big companies. I am really not in the mood for doing a 1-year study with 10 different brands, plus I feel that the 4Runner deserves the best, sue me .

The point of this post? Just a 'lil update and telling everyone that I haven't forgotten about this thread, especially because many people are more concerned with their MPG now with rising gas prices.

Since it's so late, I am going to log off. Tomorrow, I shall post screenshot proof of my MPG results and possibly edit this post because I am so tired, all of the above might not make any sense. But then again, it might!

 

My old '99 LTD (110K miles) didn't have a good gas guage, so I did the same thing!.. Excel spreadsheet and all. I got about 16.5 mpg. I wrecked it a few weeks ago and got another '99 LTD... this time w/ only 60K miles... and though the guage works fine, I tried it again, and I'm only getting about 14.5mpg. The first thing that comes to my mind is tire pressure -- which I haven't checked once yet.

Also, the ratio of city:highway makes a huge difference. My loaner (between the 4runner's) was a Jeep Liberty with a fancy computer. Hwy was around 17 mpg, but w/ a light touch I could keep it over 20mpg. In the city, I couldn't get over 7 or 8 mpg. I know these are different cars, but still interesting. Driving style and city/hwy ratio definately make a big difference.