what does the ninetenths that follows the price of gas stand for ?

Answer 1

Nine tenths of a cent. It was a marketing tactic back in the old days. 24.9 sounds cheaper than 25. Of course, now it's 249.9, but I guess that still sounds cheaper than 2.50

Answer 2

It means nine tenths of one cent. Unless you get 10, 20, 30, etc gallons of gas, then it is rounded up to the nearest cent. If you get exactly 10, 20, 30, etc gallons of gas, then you save 1 cent for each 10 gallons of gas. It is simply a way of making gas look cheaper, and seeing how prices are going these days, cheap is good.