Full Tank of Fuel = Higher MPG?

Sunin · #1 10-05-2009, 10:02 AM

First I just finished my first ever 800.6 mile trek on a single tank.

On a tank that was sucking fumes I was struggling to get 64.1 mpg just before the fill up. I had noticed on the high way I could only maintain 52 mph and maintain 75mpg.

Fast forward. Full tank later and I struggled to get it under 70mpg. On the high way I was able to keep 57 mph or higher at 80mpg.

I've been keeping track of this for a while and it really feels like there is a trend for better fuel economy on a full tank. So why is that?

1. Could the extra fuel lower the car, thus decreasign drag?
2. could the extra fuel be creating higher pressure from the fuel pump

or could it simply be:

3. Different winds
4. Different fuel

I must say it does not seem to be 3 and 4 as I've noticed this over the last 5k miles. I first thought it was my tires going away as the tread is declining, but then I take it out yesterday and easily hit above 70mpg. Also on the same tank over time I see the fuel economy drop off from 70 to 64 throughout the fuel load. It seems pretty linear to the amoutn of fuel in the car. Again my perception.

Any clues as to what I'm perceiving?

09Ranger2.3XL · #2 10-05-2009, 10:29 AM

I have wondered this myself.

Could it be when it's full that there is less air in the tank, thus allowing less evaporation??

Just a thought.

Right Lane Cruiser · #3 10-05-2009, 10:52 AM

I haven't experienced this -- other than right after a fill when the iFCD gets really optimistic (due to scavenging fumes from the fill?). All other factors being the same, my mileage tends to increase gradually as I near the end of the tank.

PaleMelanesian · #4 10-05-2009, 10:57 AM

Mine improves as the fuel level drops, by about 1-2%. That's right in line with the weight reduction from the empty tank, also 1-2%.

* This holds true down to about 1/8 tank, and then it falls like a rock. I blame fuel pressure for that, since it gets harder and harder to start the lower it gets. On a nearly dry tank I can actually hear the different tone from the fuel pump priming - it's not getting up to pressure.

09Ranger2.3XL · #5 10-05-2009, 11:11 AM

I can't wait to get my sg so I can track everything better & see what works & what doesn't

Nevyn · #6 10-05-2009, 11:15 AM

I bet it's a fuel pressure/engine vacuum thing. My car exhibits similar behavior; however, comparing it to an Insight is like comparing a bicycle to a bazooka.