# Does the time of day affect fuel economy?

One tip that I heard as a young driver was something along the following:

Try to refuel in the early morning or in the evening as the denser gas will get you better fuel economy.

Since then I have seen the same advice or question appear online from time to time. As such, I have been wondering, does the time of day that you refuel have a meaningful (even if over the long run) impact on a vehicle's fuel economy?

• Temperature affects the density of the fuel; which matters in places that sell fuel by volume. There is such a thing as mass flow meters, but they are not used in many countries. – dmckee --- ex-moderator kitten May 14 '12 at 2:12
• It's an odd phrasing of the claim; I suspect you misunderstood. I've heard it as you will get more fuel mass per litre, so the fuel is cheaper \$/kg. But that's not related to fuel economy. – Oddthinking May 14 '12 at 2:46
• If be fuel economy you mean currency/distance (or the inverse, of course) or volume/distance then it does matter. Of course mass/distance makes more sense. Of course, the conventional units used in the US and Europe are both in terms of distance and volume. – dmckee --- ex-moderator kitten May 14 '12 at 3:11
• @dmckee: That's not "fuel economy," though... that's just plain "economy." :) – Flimzy May 14 '12 at 3:13
• @dmckee - I believe that the currency/distance model is what is at the heart of the claim. – rjzii May 14 '12 at 5:22

You can expect a density change of 0.7 g/l/Degree:

Density of fuels in g/l with respect to temperature; red curves represent the tolerance band for gasoline; blue curves represent the tolerance band for diesel fuel and the pink curve represents the density of pure ethanol

Source

So, if you consider a temperature span of 20 C during the day, your density variation will be near 15 g/l (or 2% = 15/700) ...

Edit

Two interesting related articles:

Snopes: Tips help consumers get the most from their gasoline dollar

LA Times: As temperatures rise in underground storage tanks, gasoline loses energy, but fuel prices don't flex to compensate consumers

• 15g per litre is more than I would expect - with a 50 l tank is gives you almost a litre for free. However: 20 deg difference is quite lot, and moreover, this assumes the fuel temperature will differ that much as well. Given fuel tanks are usually underground, I am afraid the real difference will be much smaller. – Suma May 14 '12 at 13:17
• @Suma There are also a lot of other factors to consider. For example, the fuel is usually transported (in my country by truck) to the gas station. Thermal equilibrium after loading the gas station tanks may take quite a while – Dr. belisarius May 14 '12 at 13:25
• @Suma - Depending upon the time of year and where you live I could see there being a 20 degree difference (Fahrenheit) over the course of the day. – rjzii May 14 '12 at 13:52
• @RobZ 20 deg F would be only 11 deg C. 20 deg C is possible, but very unusual in normal inhabited areas. That said, I consider the second part of the argument (underground tanks) more important. – Suma May 14 '12 at 14:03
• Even if the air temperature were going through 20° swings, I would not expect the temperature of the fuel stored in underground tanks to very anywhere near as much. – stew May 15 '12 at 23:27

In Canada fuel prices are by law corrected for this effect. Here is what the Consumers Council of Canada says:

Gasoline, like everything else, expands as it gets warmer and contracts as it gets colder. Without temperature correction, a litre of gasoline on a warm day would contain less gasoline by weight than on a cold day. Gasoline is sold by volume but what you are actually paying for is the energy to make your car go. Temperature correction ensures that, regardless of temperature, you receive the same amount of energy for your purchase although the volume may differ. Measurement Canada, an agency of Industry Canada, regulates measurement of gasoline at the pump.

• Thus getting around the problem of already having a bjillion (well, at least 100 because this is Canada...) volumetric flow meters already installed. – dmckee --- ex-moderator kitten May 14 '12 at 20:25
• Does my Mum's family being from BC allow me to make that kind of joke? – dmckee --- ex-moderator kitten May 14 '12 at 20:26
• No problem. Everyone knows we only use gasoline to heat our igloos anyway. – DJClayworth May 15 '12 at 16:00