“it’s a good idea to live more than one gas tank away from any large city.”
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Good luck with that unless you mean VERY large city. A tank of gas is a one way trip of four hundred miles or even more depending on vehicle.
Probably 300 miles is good enough. My gas tank holds 20 gallons, I supposed big trucks might have bigger ones. I have two old cargo vans and usually if I drive to the nearest big cities (both about 6 plus hours away) I generally need to refill at some point in the journey.
Even 200 miles is better than nothing. But to live within half a tank of any big cities is IMO something that will be later bitterly regretted.