why does a gallon of gasoline always have 9/10 if a cent at the end of the price?

That's part of the Tax scheme. Read further or go to your States website and see what the tax is on a gallon of gas. Then think about the roads and see if you think the tax money is doing a good job for your state.

Marketing. It goes waaay back in time to the first psychological profile of a customer. People remember the big numbers, so we end up dropping the "change". It also ties into "rounding up or rounding down". If you want to sell an item for $9 you could make it one nickel less and people would see it for $8, perceiving it to be a greater value.

I think it's so that it will appear to be cheaper at first glance than it really is. That may be totally wrong, but that's a good question...I have often wondered.

Discussed a lot here. http://depts.gallaudet.edu/ESL/cloze13r.htm

Bottom line, it is a marketing ploy. $23,999 sounds cheaper than $24,000.

And $1.999 is cheaper than $2

they always do that for no reason just to make it appear cheaper but it really isn't