Refill a empty tank or half full

[jakobnev]

Maybe you have a leak.

[Ecky]

Originally Posted by ST2008

It's what I did every time. Same gas station, same pump, two clicks, same path, same speeds and etc. I always compare only with freeway trips because the speed is almost always constant.

What do you mean you compare only with freeway trips? Do you not keep record of every fillup?

To me, typical driving habits, not just my best days, are important.

[ST2008]

Originally Posted by Ecky

What do you mean you compare only with freeway trips? Do you not keep record of every fillup?

To me, typical driving habits, not just my best days, are important.

Because freeway trips have less immeasurable variables and I want to be sure that the only variable that causes my inconsistent mpg is the quantity of gas in the fuel tank.

[ST2008]

Originally Posted by jakobnev

Maybe you have a leak.

Probably. but so far, It doesn't smell gas and I don't see any leak in my driveway. But I'm going to examine the fuel lines thoroughly next time when I jack the car.

[niky]

Originally Posted by ST2008

Not sure if you're serious or sarcastic. If you're really serious, could you explain more about your ABABAB test ?

For myself, I already verified several times with half empty and almost empty tank. The almost empty tank has always much better mpg.

Simply the way you're doing it... but half-fill to full-fill to half-fill to full-fill... to try to minimize variables. Of course, with a full tank being between 400-600 miles, that's problematic, as there's a lot of variability over a full tank.

-

From what you're saying, you're getting almost 600 miles out of a tank, and when you fill at half full, just over 200 miles? Would help if we saw the raw numbers, and the dates of the fills.

[Ecky]

My car has a complex fuel economy display, and I know exactly what my fuel economy is at any given point. When I pull into the pump, I know what the tank is going to take within a tenth of a gallon, whether I fill it from 3/4 or empty. Fuel economy doesn't change a bit from tank level.

Something like a Scangauge or Ultragauge can give you average tank fuel economy, in addition to instant MPG. You can do that with Torque too, actually, but it requires that you are connected to the app 100% of the time you're driving.

[JRMichler]

And let's not forget the other variables - temperature, wind, road grade.

My mileage changes almost exactly 1 MPG per 10 degrees F, 1 MPG per 5-10 MPH wind speed, and about 2 MPG for an elevation change of 11 feet per mile. My gas log shows the average effect of temperature change very nicely.

[ST2008]

I think the difference is to high that it was caused only by the external variables.

I actually find this by accident because I usually refill my tank between 1/2 and 1/3, when the gas price was dropping. I was also satisfied with my "usual" 35mpg.

But three weeks ago, I had a very long trips with my car and I took this opportunity to see what was the "precise" mileage I can get with a "full" tank. To my surprise, I was able to reach 750km with 42L (95%HWY, 5%city). I was sceptical at first, so I did lots of "refill" next two weeks to see what is the " average " mpg of my car. But I was unable to produce the same or near the mpg with the full tank.

So to make sure I'm not hallucinated, I decided to recheck my mpg with another long trip.This time, I brought few measurement gadgets with me and avoided as much as possible immeasurable variables. For these:
- I brought the torque pro with me for average and live "mpg".
- Avoid as much as possible city driving (2%)
- Same gas station, same pump and two clicks
- Use the same roads for the return trip
- Same travel speed 60mph

This time, I was able to produce the same result (765KM/41L)

So far, I was unable to find what cause this big mpg difference between refilling an empty tank and a half full tank. This is the reason why I ask you guys if some of yours experience the same situation.

[Hersbird]

If this were true, it would mean the mileage from 1/2 tank to empty to was even better, over 50 mpg. The mpg from full to 1/2 will be the same whether or not you refill at empty or 1/2. Basically the car would be getting 35 MPG from full to 1/2 and then 50 MPG from 1/2 to empty for a 44 MPG average. If that were true, I would refill at empty but only put in 1/2 a tank. I bet there is something else wrong with the calculation or even a leak in the full tank.

[ST2008]

What I'm going to do next step is to find somewhere a very accurate vehicle scale. With this, I can calculate my mpg without refilling the tank.