why would the car leak gas by the back driver side tire when I start it?

Hello, strongly recommend a professional evaluation, as this is a fire hazard. You could have a restricted component of the evaporative emission system, you could have an issue at the filler neck. Or something else! Check this link for folks near you that can help: http://repairpal.com/kia-within-10-miles-of-phoenix-arizona-85041