When to refuel
 

I searched for this but did not find what i was looking for.

Was talking to a co-worker today - had mentioned the ECO driving and how well i am going with it - he mentioned to me that the best time to fuel your car is 1st thing in the morning with your tank no more then 1/2 empty ( or was it 1/2 full :rolleyes: )

He went into some longwinded explanatuon about the fuel vapor pressure and the additional "liquid loss" after the tank crosses the 1/2 way mark as "gallons you can't use" :confused:

Then he told me me how the 1st thing in the morning - the liquid is more dence in the colder temperature - thus the volume of a gallon is less :confused:

the first one i could almost follow him - almost made sence - except for the Emissions testing we undergo in Georga - they check yoru gas cap for proper seal - so the Vapor can't escape - so is it really lost?

The second point just seemed plain wrong to me - may be it was his argument - but could not see how temperature could change it that much.
( would your swimming pool "overflow" in the afternoon if you filled it up in the AM)

Anyway - i wanted to see if either of those had been discused - I could have missed it when i searched for them - and if this has been discussed - My apollogies for the repeat of the subject

Thanks
Steve

IMO the temperature difference is so tiny it won't matter in the least bit. Basically, the theory is that during the day it gets warmer, so the gas you pump into your tank is warmer and less dense. So, you get more gasoline for the same amount of money. I don't think this theory is very sound as the gasoline is kept in tanks that are underground. There is no way that tank heats up much at all during the course of one day. Week to week maybe there is some variance, but not day to day. Also, the time it takes to pump the gas from the tank to the tank of your car is not much time for it to warm up and expand either, and thats not to mention that the lines are pretty well insulated.

As for the vapor, you are correct. Modern vehicles have vapor recovery systems that feed gasoline vapor from your gas tank to the intake manifold to be sucked into the engine and burnt. Everytime you open up your tank to the atomosphere you will loose some vapors no matter what. But, the amount is so small you will never notice. I don't see how filling up at less than a half tank would make much of a difference other than allowing more area for vapors to occupy. Since they are burnt, its not an issue.

And the car is lighter with less gas in it,
about 5 to 7 pounds per gallon.

today i inquiored with the co-worker to better explain is reasoning - i played the "dumb engineer" asking questions he thought were Obvious.
Needless to say - he could not really explain the points he had made

I think it was a case of a Smart Guy trying to fill in the blanks to sound even smarter.

I always try to fill up at 1/4 tank (ish) only cause you never know when the kids will have some emergency and that is not the time to get gas.

Steve

Also, fuel injected cars use fuel to keep the fuel pump cool. Letting the tank run low allows the pump to run too warm, and wil cause it to fail.

I just wait till its below 1/4 or so then I'll have time to plan ahead for a stop at the same pump at the same station every time...trying to keep the scangauge calibrated.

Personally I have always kept the tank at half full or more to avoid sucking up the water and other rubbish which inevitably accumulates at the bottom of the tank.

Tests done here by a consumer magazine regarding the filling of the tank at different times of the day found there was no difference in the amount pumped out.

Pete.

Since gas floats on water... The fuel pickup is in the bottom anyway, and they USUALLY not always, have a filter/sock to keep from picking up big stuff that could clog a pump.

I'm pretty sure gas pumps use mass flow rather that volume when they meter out gas. You are probably right about minor variations in volume with changes in temperature, but they are variations nontheless.

Then again, many pumps are probably calibrated incorrectly (unintentionally or intentionally)...

- LostCause

Every thing you wanted to know. It's a little dated but you'll get the general idea.