A Full Fuel Tank and Gas Mileage

courtesy of Bob, The Auto Answer Man

Petrol prices here in England are very high in comparison to any other Western country. With this in mind, when occasionally prices do go down it would make sense to fill up the tank.

However, I have heard two different things that prevent me from doing just that. First, I was told that petrol 'evaporates' from the tank, and second that car with full tank is less fuel efficient - being that it's heavier and thus uses up more. I don't drive much or often - most of my trips are under 10 miles and occasionally I go for away for a long weekend. What would you recommend? Should I fill up (it also saves me time) or visit petrol station fairly often regardless of prices? Many thanks.
S. H.

The general rule of thumb is that for every 100 pounds of weight you carry, you lose 1 mpg. Gasoline weighs about 7 pounds per gallon (I know you are in England and your units of measure are different, but you will see my point). If you fill up with about 15 gallons, that would weigh in at 105 pounds of gasoline. So, theoretically, with a full tank of fuel, you will be getting 1 mpg less than with an empty tank. However, you won't get very far on an empty tank of gas. I would say fill up when the price is low.

You mentioned the fact that the fuel evaporates. Well, this is true, but the process is not that fast in a sealed system like your fuel tank. How often do you go to the fuel station? If it is more frequently than once every 6 months, then you should be fine. Plus, the fuel doesn't stagnate because you stated that you drive about 10 miles a day. That should be sufficient to keep everything mixed nicely.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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