When to Fill up
 

I know I have seen threads that give the best time fill up ... empty vs 1/4 etc ...

However has anyone determined whether its best to fill to half a tank all the time or refill at the half way mark? Law of averages aside, I was wondering more about gas evaporation and whether or not gas evaporates faster in a tank that is half empty with obviously more agitation or mostly full, with less room for agitation.

Thanks ...

Gas doesn't evaporate from a modern vehicle. Fumes are captured and burned in the engine via the evaporative emissions system. So I don't think I'd worry too much about that.

Good to know, thanks for the knowledge, now I can finally run a tank down and reduce the weight of half a tank ....

Dont know exactly how the evaportaive emissions system works, but from a strictly theoretical standpoint, less gas in the tank will increase the vapor pressure of gaseous gas (more gas will be in vapor form in a half a tank than a full tank). From that point i guess you would have to figure out which system uses the gas more efficiently, the conventional system or the evaporative system

Also running the tank low constantly allows an electric (in tank) fuel pump to pick up garbage and debris and may shorten fuel pump life.

That concern is almost eliminated in these new compacts as they have sealed plastic tanks with no drains .... Fuel filters pick up any additional impurities.