why would my gas mileage change based on how I fill up?

Discussion in 'Other Auto Tech' started by mfp4073, Dec 30, 2007.


  1. mfp4073

    mfp4073 Founding Member

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    This is in my 94 mazda mx6. 2.5fi manual 5 speed with overdrive and regular distributor.

    As far as I can tell, if I run an entire tank of gas out and check the milage I will get about 1.5mpg less than if I fill up every time I get to a half a tank. Usually mpg is about 23 if I measure it from a full tank. I get 24-25 if I keep the tank from dropping below a half a tank. This makes no sense, because you would think that the extra weight of the gas would make the milage worse.....been testing this for several months now.....
  2. Therian

    Therian The Highlander Founding Member

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    Might want to run a tank of ethanol blend through til it's almost empty. It may be that you have a lot of sediment (and possibly water) in the bottom of your tank and it may be clogging your filter near the end of the tank (decreased fuel pump pressure + added particulate matter).

    In the winter it's easier for water condensation to accumulate in your tank (especially when it is below half full) and this can affect MPG.
  3. mfp4073

    mfp4073 Founding Member

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    well, winter in florida does not really count as winter:rolleyes: ...I would be more worried about monsoon water entering the tank! And you would think that if there was a reduction in fuel pressure I would actually be running lean and have better gas milage and misfires. Runs just fine (when its not breaking down that is):nonono:
  4. jcp123

    jcp123 New Member

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    Hard to say, except that it sounds like some pump shutoff discrepancies. Do you use the same pump at the same station every time? I presume you pump same speed every time, and finish it off the same way every time? Maybe look at the big picture - do your half-tank usages roughly coincide with warmer weather than your full-tanks?

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