Seriously? Are people this dependent on other people and the internet that they cant figure out a basic thing like that?
If its your car, you should know what sort of cost per mile as second nature. If its not, a quick google to find out what sort of economy to expect in miles per galon. You know what a litre of fuel costs so you know what a galon costs - you know how many mpg and how many miles so you can easily work out how many galons you need, and thus what it will cost.
Its GCSE level maths, if that.