Odd Bumps in Mileage
 

I have noticed a trend in my last 5 tanks that has me scratching my head. My MPG's are jumping between 39 & 41 MPG. Alternating with each tank. And I mean literally 1 tank 39, next tank 41, next tank 39 ... etc

I use the same pump and station each time and even fill at the same time within 30min. Any guesses as to what gives?

Maybe when you get the 39 you are motivated to get better mpg...then you get the 41 and relax a bit...back to the 39...motivated to get the 41...magic!

The automotive equivalent of yo-yo dieting

Sorry bro...only thing I could think of...and probably no help to you

check for a young neighbor kid with a siphon hose

Based on your fuel log for your vibe and ignoring the single sub 1 gallon fill, your average fill is ~2.8 gallons. With such small fill amounts there is bound to be a significant variations in yourFE averages.

What fuel logs are you looking at??? My fills are never less than 9 gallons ...

TestDrive I aggree with him, I don't know where your getting this "average fill is 2.8 gallons"???

Sorry, no idea what I clicked on to get the completely wrong info. Just ignore.

Possible fluctuations in weather? How are your scangauge readings, do they alternate the same or are they steady? You drive enough miles with enough consistency where you probably know at any point in your drive what you normally get and if its high or low.

I have to say, all reading are consistent, my scangauge reads a consistent 41-42 mpg avg daily on each side of my commute, and the mileage markers I have for the 3/4, 1/2 are usually very consistent, its only until I get to the 1/4 tank line that I see a larger fluctuation.

You'd think that as the car got lighter it would do better. As far as weather conditions, this is S. Florida, this time of year its high 70's in the day, low 70's at night. I always have to contend with 10-15mph crosswind heading in both directions because the wind pattern here is usually east to west and I drive North to South, etc.

It is puzzling, but I'll give it a couple more tanks before I go a little deeper into this problem.