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Old 05-07-2008, 06:59 PM   #36 (permalink)
MPG Centurion
Join Date: Apr 2008
Location: Houston, TX
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Originally Posted by MetroMPG View Post
Thanks, Dan - I'll add it.
Very Cool. The digg count is going through the roof on your tip list. My google alerts on hypermiling is getting hyper too. Great job!

Originally Posted by MetroMPG View Post
I wonder about the cold gas tip though. Not questioning the physics, of course. But is it more of an economics issue than an efficiency issue? The difficulty is how do you choose to buy cold fuel vs. warm? (My reading on the late day vs. early morning fill timing is that underground tanks are mostly unaffected by atmospheric temp fluctuations.)
Your right on target. The "tip" doesn't reduce your tail pipe emissions, but it does reduce the amount of money you spend on gas each month. Think of it as cold gas costing less than warm gas. The tip can be summarized into Always fill up in the morning, never in the afternoon. Now as far as "what is the temp delta in an underground tank", depends. In Houston, the tanks aren't really underground, but rather in-ground. Think of a back yard swimming pool with a tarp over it. That tarp is made of concrete and has pumps on it. Drill 6 inches through the concrete and your in the stations storage tank. So yes, sun hitting the concrete all day will heat the "roof" of the stations storage tanks, and through convection, the gas itself.

The brainchild of this tip was a law passed in Canada (I believe) a while ago. Basically gas trucks were getting short changed in the winter. They'd fill their trucks to make their deliveries. They would fill 9000 gallons at the distribution center, but only sell 8950 gallons returning the truck bone dry. What was happening was that the gas would cool during the deliveries. So the solution was that tankers sell by the pound. Consumers buy by the gallon. Doesn't matter if the gas in a 9000 gallon (sealed) tanker is warm or cold. It's weight never changes. The fact that the tanker drivers had to push this law through got us thinking that it must be a big enough volume change to warrant all that effort.

Here's a source (the state of Utah):

Google keyword list {gas density morning}:

Here's a counter-source that claims that all US pumps use temperature compensating volumetric flow meters.
I can't confirm this claim:

Houston Hybrid and Hypermilers Club <>

Best commute = 14.3mi @ 114 MPG (sg2)
Best (non-trivial) tank = 759mi @ 80.7 MPG (fcd)
MPG Centurion-Hybridfest 2007-Prius II-26mi @ 106 MPG (sg2)
Dan <11011011>
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