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Does anybody know if there is a reserve amount in the fuel tank after the computer calculates 0 miles left on the tank? In all the other cars that I've owned, when the distance to empty reaches 0, there's always an extra amount of gas in the tank (usually 1 gallon) to allow for more driving.

When I've been filling up my XC40, I've noticed that the distance to empty and the amount that I fill, knowing it's a 14 gallon, tank makes me think there's no extra reserve. This isn't ideal since the distance to empty on this car fluctuates so much throughout a trip. I've taken a a 35 mile trip and the distance to empty would fluctuate from 80 miles to 20 miles to 40 miles.
 

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The MY16 XC90 miles to empty algorithms were updated early on and tweaked often as some people were running out of fuel - relying heavily on the miles to empty.

Now, the XC90s account for about one gallon of a reserve - at least those with a 13.3 gal. tank. Larger tanks may have a larger reserve. Hopefully, that is the case for the XC40 and all newer models.

Personally, I would never rely on miles to empty since driving techniques, terrain, wind, and traffic will affect such a computation (which is an estimate at best and an average at worst).

IMO, tracking the estimated volume (gallons/liters) is more reliable since we humans can apply our own average (worst) case to the volume remaining to help mitigate the risk of running out of fuel.

I would much rather have a digital “gallons/liters remaining” feature (while in steady motion and on flatland).

Here is what the Volvo FAQ says:
Understanding Distance to Empty
Distance to empty calculates the approximate distance you can drive with the amount of fuel remaining in the tank. This calculation is based off of the average fuel consumption over the past 20 miles of driving along with the amount of fuel left in the tank. If you see the symbol "----" you need to refuel as soon as possible or you may run the risk of running out of gas.

Note: The accuracy of this information may change based on your driving style. Once you refill your gas tank and reset your MPG, by pressing and holding the reset button on the end of the left hand steering wheel stalk, it will take approximately 20 miles of driving to provide a new accurate distance to empty reading.

An example of how it functions:
If you have a full tank and are driving on the highway, your distance to empty will show a greater distance than when you switch to city driving since the driving requirements for city and highway driving are different.


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Also, for what it's worth, many mechanics and car manufacturers recommend not letting your gas tank get below 1/4-1/3 full primarily to insure proper operation of the fuel pump and fuel filter(s). In the case of the fuel pump, as many pumps these days are inside the gas tank, the pump is engineered to be gas-cooled. In terms of the filter, too little fuel would increase the odds of tank debris being sucked up and potentially reduce the pass-through of "clean" fuel. Just a thought.
 
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