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According to this site it is possible to get more diesel fuel in your tank by pressing in the vent latch on the filler neck:

Look inside the filler neck and you can see a button, see below for detailed pics. Press on it when the tank is almost full and you will hear the hiss of air venting. This lets you add about 2 extra gallons of fuel. Venting does not increase fuel economy at all, it just lets you add a little more fuel to increase miles per tank.

Also

Heated diesel expands much less than gasoline so the likelihood of overflowing fuel is very low

Is there any evidence that these claims are true and is putting more diesel fuel in in this way safe?

Update:

Seminecis's answer is good, but are there any similar regulations in the UK?

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  • I updated my answer with what I could find about the UK. I couldn't find official legislation. It still sounds dangerous. Hope it helps.
    – Seminecis
    Aug 30, 2013 at 18:56
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    It's not clear why this is something that you'd want to do. You're trying to increase the distance between fuel stops? If so, this sounds like a lot of work to defeat a venting system when you could get the same effect by driving slower and / or with more relaxed starts from stoplights.
    – Bob Cross ♦
    Aug 31, 2013 at 17:41
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    @BobCross A lot of cars in the UK have small fuel tanks: e.g. carpages.co.uk/guide/volkswagen/…, so this would increase the range. The modification does not have to be done, it is possible to use the expansion tank by depressing the latch with the nozzle each time instead.
    – SilverlightFox
    Sep 2, 2013 at 9:10
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    That link suggests the car in question has a 9.9 gallon tank and does 72.9mpg combined - that's an estimated range of over 700 miles, or nearly a month's usage for the average UK driver... Is it worth the danger of a fuel spill to get an extra few miles between fills?
    – Nick C ♦
    Sep 2, 2013 at 12:55

3 Answers 3

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Despite any opinions of safety it seems that gas tanks are manufactured to not accept more than 95% of their total volume because of regulations. Here is a quote from the US Department of Transportation Federal Motor Carry Safety Administration Regulation 393.67 Subpart E.

(12) Overfill restriction. A liquid fuel tank manufactured on or after January 1, 1973, must be designed and constructed so that—

  • (i) The tank cannot be filled, in a normal filling operation, with a quantity of fuel that exceeds 95 percent of the tank's liquid capacity; and

  • (ii) When the tank is filled, normal expansion of the fuel will not cause fuel spillage.

Chevron's Diesel Fuels Technical Review - page 34 says:

THERMAL EXPANSION Like all liquids, diesel fuel expands slightly in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion. A typical value of the coefficient of thermal expansion for diesel fuel is 0.00083 per degree Celsius (0.00046 per degree Fahrenheit). Using this value, 1.000 gallon of diesel fuel at - 7 °C (2 0°F) will expand to 1.037 gallons at 3 8°C (100°F).

... meaning if your tank has a capacity rated at 25 gallons it is possible for it to expand up to 25.925 gallons in the described scenario. Meanwhile, a tank designed to hold 25 gallons of fuel must be designed to be at 95% capacity maximum, which means a tank that can hold 25 gallons will actually be a 26.31579 gallon tank to allow 5% for expansion. That means that in the above scenario there would be 0.39079 gallons of empty space left after expansion.

Though that scenario seems extreme, it is possible to encounter even more extreme situations even if it is unlikely. I wouldn't take the chance of overfilling the tank since it sounds dangerous and modifications described on the website you linked to might even be illegal.

EDIT1

I couldn't find official regulation pertaining to the UK but, there is a guide at gov.uk that says:

Diesel spillage is dangerous to other road users, particularly motorcyclists, as it will significantly reduce the level of grip between the tyres and road surface. Double-check for fuel leaks and make sure that - you do not overfill your fuel tank

Whether or not it is legal to overfill a fuel tank it is still dangerous in practice.

EDIT2

I was curious and did a little more research on the differences of thermal expansion between gasoline and diesel fuel. I found the coefficient of thermal expansion for gasoline on answers.yahoo.com and wikipedia.org.

                Diesel        Gasoline
coefficient    .00083        .00095

Same -7 Deg. C (20 Deg. F) to 38 Deg. C (100 Deg. F) temperature increase as above.

1 Gallon    1.03735 G        1.04275 G        Difference of 0.0054 G (~= 4 tsp.)

Same 25 Gallon tank as above.

25 Gallons    25.93375 G        26.06875 G        Difference of 0.135 G (~= 1/8 G)
% of total    3.735% Expansion    4.275% Expansion    0.54%

Which I take to mean that the author of the article you mentioned is technically correct that diesel expands less than gasoline but, he is very inaccurate describing diesel as expanding "much less" than gasoline. Looking at the comparison objectively, I would say that the thermal expansion of diesel and gasoline fuels are comparably similar rather than noticeably different. In fact, I would interpret the numbers as: "they both expanded one gallon in the 25 gallon tank."

I guess I'm trying to say that I don't fully trust the author of that article.

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    great answer! This is not that extreme at all, in colder climate you could fill your gas tank way below 0°C (let's say -40°C) and enter a heated garage at 25°C just after, that could cause a spill if the tank is full. Also, the filler neck could hold some of the overflow, but not all.
    – Gabriel Mongeon
    Aug 30, 2013 at 15:16
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I do not recommend modifying your fueling system due to the extensive design that goes into these types of applications. The tank, along with the supply and return systems have been designed to not only accommodate 5% expansion space but also to adequately supply fuel to your engine as energy and lubrication. In diesel applications, hot fuel is also returned to the fuel tank during normal operation. This flow rate is less than the supply, but the temperature is higher which, like various ambient temperatures, causes expansion. Since ambient temperatures widely vary between locations such as Canada and Mexico, yet the same manufacturers readily supply diesel tanks globally into various applications (on high-way vehicles/agriculture/generators/etc), regulations effectively accommodate these variables. Unless you fully understand your engine and all ancillary systems including the canbus system that runs your engine, I don't recommend modifying anything....Especially on an on-highway application that is susceptible to strict emissions regulations.

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The 95% volume is a regulation due to fuel expansion. If you pump fuel from ground tanks (pretty typical) the fuel is coming up the temperature of the ground (45 degrees) and if it is a warm day, the fuel will expand as it warms. Say you drive in a southern environment and the fuel warms to 100 degrees. Every gallon of fuel expands to 1.0253 gallons.

Due to this a 100 gallon tank is supposed to have 5 gallons of expansion volume. The most extreme temperature changes would be in northern environments (where I live). It is rare, but possible for temperatures as low as -40F. So a truck is filled with above ground tanks with -40F fuel, and then moved into a heated shop for maintenance at 70 degrees. That is a 110F degree temperature swing. Using that 100 gallon tank again, the 100 gallons of fuel would expand to 105.06 gallons.

This is why it is not recommended to force more fuel into your tank than it accepts as designed.

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