Answer
$10^{11}gal$
Work Step by Step
There are about $3\times10^{8}$ people in US, a reasonable estimate of the number of automobiles would be $1.5\times10^{8}$.
There are 365 days per year.
Average gasoline used per day is greater than 1 gal, but less than 10 gal, let us take 3 gallons per day.
Estimating hence,
gal = $1.5\times10^{8}\times365\times3=1642\times10^{8}=10^{11} gal$
