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I was filling my car earlier today, and noticed a sticker posted on the pump.

This pump dispenses fuel at a volumetric amount measured in standard gallons (231 cubic inches). It does not adjust for temperature, or other factors that may affect the energy content of each gallon.

This got me thinking.

  1. What would be the optimal conditions to fuel your car so that you can get the most energy per gallon? Let's assume we measure this in miles per gallon achieved traveling at 60mph in a standard car that advertises 30mpg; a Honda Civic, perhaps.

  2. Why would temperature affect the energy of each gallon?

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As you are charged per volume and the density increases with decreasing temperatures it is better to fuel when it is cold. In practice is does not matter though, as the fuel comes from an underground tank which stays at almost a constant temperature. – Alexander Nov 29 '12 at 23:20
@Alenanno to state that one is not sure about the tags and needs help in this is NOT an unnecessary comment since it is important that a question has the right tags in order for the right people who could be interested in the topic and give an answer to notice it. – Dilaton Nov 30 '12 at 0:00
Can someone please take a look at the tags? The user was not sure about which one(s) to use. – Alenanno Nov 30 '12 at 0:46
@Dilaton true, however such a comment belongs as a comment, not in the question. So it is always OK and encouraged to edit such statements out. – David Z Nov 30 '12 at 22:40
up vote 2 down vote accepted

Why would temperature affect the energy of each gallon?

The energy content depends on the mass (i.e. on the number of molecules available for combustion)

The volume of a kilogram mass of gasoline depends on it's temperature - gasoline expands and becomes less dense as it gets warmer. So a litre of warm gasoline contains less mass than a litre of cold gasoline.

The difference is very slight and underground gasoline storage tanks maintain a fairly even temperature day and night. The difference may be less than the volumetric accuracy of the gasoline pump.

When gasoline is delivered in large tanker-trucks, the temperature is taken into account when calculating the value of the delivered volume.

What would be the optimal conditions to fuel your car so that you can get the most energy per gallon?

As explained above, in practice this isn't worth doing. A better strategy is to drive smoothly, plan ahead whilst driving, use throttle and brakes as little as possible, use highest possible gear, don't use high speeds.

An overly literal answer might be:

At the end of your previous journey, remove the fuel from your car and put it into a chiller. Just before your next journey put into your tank just sufficient mass to complete your journey. By using chilled fuel, the volume is less so the number of gallons is fewer - this however does not save money or fuel (as measured by mass) it just gets you furthest using the least number of gallons.

By not carrying fuel you won't use, you reduce the mass of the vehicle and it's contents, less mass means less force needed to achieve a specific acceleration, which means a reduction in fuel consumption. This doesn't increase the energy per gallon but it does save you money.

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I wasn't planning on trying to improve my fuel economy by when I fuel. I was just curious to know if there would be a measureable difference. I guess there isn't. – Joe Nov 29 '12 at 23:38
The difference in volume is something like 1% for a 15C change. But temp. of stored fuel is not same as current air temp. – RedGrittyBrick Nov 29 '12 at 23:41
I recall some warning about fueling an airplane when it was cold (Minnesota, here), that when it warmed up, it might overflow the tank. – Bobbi Bennett Nov 30 '12 at 5:43

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