[mikhailfranco]

It's not too difficult to know or calculate inches in a mile:

    12 * 3 * 1760 = 12 * 5280 = 63,360

A few Americans may know how many cubic inches in a US gallon:

    231 

But I don't believe anyone (apart from rainman/prodigy circus performers) can do this in their head:

    63360^3 / 231 = 1,101,117,147,428.571

Estimate yes; calculate no.

[MarkusWandel]

That's probably what it was, an engineering estimate with a couple of significant digits. It's been years.