Calculating fuel tank contents.
While attempting to maxime mpg at each leg of a recent hypermiling excursion, the only way to figure precisely what's used up for each leg was to run small tank-portions empty each time.
This practise has exposed to me a difference between various fuel-grades that make positive FE difference; better fuel-grades, simply put, works much better with FASing a longstoke single cam engine with a 5speed like is in my 94 Saturn SL. For the event of miscalculation to coast up-to a pump, I carry a gallon-can with a measured amount of fuel stored safely in the spare tire compartment.
... found that when a filled tank gets warm it expands in various degrees thus leaving less volumeweight for a last-minute top-up etc to occupy despite a much higher volumeweight consumed; but also found that overnights can cool your tank which then often takes a lot more fuel volume to do that .... before leaving for Florida etc. This can be ellusive to precision fuel-measuring, going both ways ... sometimes (rarely) up to 10% more or less FE than expected just on that point alone.
Apparently, dieselfuel has the greatest weight-difference in comparing a WARM full-tank content to a COLD fill-up, the latter which yields significantly more distance.
Our VW diesel friends had discovered this expansion dilemma.... where the warmed-up fuel-fill, including that fuel still in the tank that has warmed up due to daytime or fuel-pump-return-heating prior to filling, robs that tankful of a number of lbs. This makes the last tank's FE look better .... and might make the next tank (esp if its results are with a cooler fill) look bad in the comparison.
Last edited by pdw : 10-17-2008 at 12:14 PM.