I live in Wisconsin, and every time the weather gets cold people tell me to make sure I keep my gas tank full. When I ask them why they don't have an answer. Why do people always tell me to keep my tank full in the winter? Does it have something to do with condensation forming in the tank? Is it just so there's more weight in the back in case it snows? I've been wondering about this for years. Can someone explain this to me? Thanks.

And do they tell you to keep your tank empty when the weather is hot? When everything is at the same temperature (air, fuel, metal), there is no condensation (hot or cold)
fairy tail, urban legend, myth, folklore.

I'VE been told condensation of the tank's air moisture will freeze and block the fuel line.
the more full the tank,less air less moisture.

one would assume if the car is warm up the inside of the car's temp is different from the outside.

on extremely cold days,my car window also ices up from condensation on the inside.(when i lived in manitoba).Can the same thing not happen inside the fuel tank?

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Tom 2010 C250 4-matic Sport (Canada)

Petaling:
Condensation only happens when water vapor is in contact with a cold object.
Air moisture in the tank (if any) is at the same temperature as the fuel, the fuel tank and outside air. There is nothing to heat the fuel tank or its surrounding.
Condensation in the inside of your car window comes from the fact that the inside air (including moisture from your breathing) is warm as compared to the glass.

You mean sunoco's fuel with ethanolm PetroCanada's winter gas,and gas line antifreeze are all scam??????

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Tom 2010 C250 4-matic Sport (Canada)

The fuel in the tank is constantly being recirculated. It heads for the injectors and is returned to the tank at a pretty good flow rate. The fuel on the way back is warmer then the fuel on the way out. It is conceivable that the tank is 20 degrees warmer than the ambient air. This would be especially true if the level of fuel in the tank is low.

I wouldn't be too concerned about it if you consume a tank of fuel within ten days or so. But, if this time is exceeded, and the vehicle has quite a few cold starts, the warmer fuel could present condensation issues.

I don't know about all MB's but on my 84 500SEL the trunk and fuel tank is heated from the air flow through the car. I haven't actually checked the heat in the trunk but I did once check the temperature in the trunk during the summer. Inside the car the temp was 78 degrees, inside the trunk it was 88 and outside was 94. So I guess the same could be true for heating in the winter and the fuel tank is in the trunk. On a long drive, the fuel could be quite a bit wormer than the outside temperature.

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84 500 SEL (307,xxx miles)

Jack(knife)d

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Tom 2010 C250 4-matic Sport (Canada)

Good point. I agree that you need a temperature differential to induce condensation.
I also believe that you need air space to allow water droplets to form from the air and attach themselves to the tank wall. So filling the tank will minimize the available air space.
So might it be that gas tanks mounted outside the heated interior and adjacent areas probably wouldn't condensate as much, if at all, as a typical Mercedes tank mounted inside the trunk?

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1986 560SL
2002 Toyota Camry
1993 Lexus