I have been seeing an interesting phenomena during the past few months and I was hoping to get some of your opinions on what might be going on.

I have an auxillary monitor that allows me view instantaneous MPG and a bunch of other stuff. After you fill your tank, you go to a screen that tells you how much fuel it thinks you have used, and you can then adjust this amount and input a "fudge factor" to make your fuel efficiency number more accurate.

In the summer, my fudge factor was -2.3%. Meaning I was using 2.3% more fuel than the computer thought I was. Then as winter came on, that fudge factor has decreased to +3.2%, meaning I am using 3.2% less fuel than the computer thinks.

Do you think the winterized fuel here in Michigan has something to do with this? I do have it verified from the fuel distributor that they winterize the fuel with an additive and not a blend of #1 and #2.

Or could it be that the motor broke in a bit more from between 25,000 to 28,000 miles?

 

More than likely its related to two things. Winter fuel that has a lower cetane, and if you are seeing significant ambient temperature differences, the flow fudge factor due to decreased temperature of the fuel. My bets on the Cetane being lower for winter fuel. You could try increasing the cetane to see if you can reverse the correction to summer.

 

I can see how the lower temps could make the specific gravity of the fuel increase. I will have to go and find the coefficient of thermal expansion for diesel and see if the ambient temp difference correlates to the change in the fudge factor.

Thanks for the reply.