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

[mfp4073]

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.....

 

[Therian]

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.

 

[mfp4073]

well, winter in florida does not really count as winter ...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)

 

[jcp123]

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?