Way to politely tell a friend NOO..!?

me and my best friend ride to work 2gether she lives maybe 4 mins away the job is 10 minutes away...we do odd jobs there 50 bucks here 190 there 70 here etc etc...well lately she's been tryna bully me into giving her crazy amounts of money for gas. she'll call and just so happen to forget she needed gas and 'the money she put in earlier didnt do anything(yeah right)' on the way back she'll say lets both put in ten for gas(she never gets the gas in front of me because she obviously isnt using her money just mine) today she mentioned everytime we get money(5 or 6 times a day) WE should take 20 and put it to the side(give it to her) i dont feel its my responsibility to fill her tank up i know i owe her something but geez..she's coming to get me tommorrow how can i tell her no without seeming selfish? (when she texts me saying it and i change the subject she'll wait til i get in the car and stop dead in her conversation example:omg how about jason texted me last night?...YOU HAVE THAT 20 FOR GAS? :-\. uuugh