This question is one of many variations of so called Fermi estimations or Fermi problems.

The point of Fermi problems is in part to show how much use can be made of commonly available knowledge by the person willing to be resourceful and make approximate simple calculations, but it is more to illustrate the difference between estimation and guessing.

The canonical Fermi problem is the question "How many piano tuners are there in Chicago?"

So, the estimations are simple:

1. Population of USA is about 300M (Population of USA)

2. Let's say each household have 4 people in average, so we have about 75M households

3. On average each house hold has 2 cars, so there are 150M cars in the US

4. Each gas pump filled twice a day and holds about 2K-3K, considering each station has about 10 pumps, we can estimat that each station can hold about 60K galons of gas daily

5. Approximate gas tank is about 80gal, so to fill all the cars in the *at once* USA we would need about 6B galons of gas

6. Bases on estimations above, we would need gas from approximatly 100K gas station to fill all the cas in the USA at once

7. Considering there are no lines on the gas stations and there is always gas available, I would add 50% to that number, so there are probably around 150K gas stations in the USA

Actually that estimate is not that far from actual number:
Number of filling stations in the USA








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