Refill a empty tank or half full

[niky]

Originally Posted by 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.

Your variable, then, is probably the length of the trip. Longer trips produce better fuel economy than a bunch of shorter trips because there's no warm-up time.

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You can check if this is it by doing a few short loops, refilling at the same tank. After the first fill, let the car sit and cool down till all the fluids and the engine is cold. Then do several loops, topping off each time. Your first fill should reflect lower fuel economy than the others.