That'll confuse the average driver. The number of people that know the various measurements for fluids is not that high. I can totally see some future social media posts excited about the $2.99 price and then getting upset when the pump shows .25 gallons.

Even if you know the measurements it would be confusing to see a sign say $2.99 for gas and it turn out to be for a quart and not a gallon when you pull up. It's assumed at this point that it's per gallon.