how bad is a loose gas cap?
 

I believe the gas cap is bad on my car. It never hisses when I open it up to fill my tank (even when it is very low). So I assume that the car is losing gas vapors. How much would this affect my mpg?

I went through that not to long ago. Here's the thread on it. Bottom line when I changed it out it still didn't hiss. No change in FE.

your cap might be good, the entire sys has pressure, it could be leaking from a stuck vent valve or hole in the hose. its a vapor recovery system if its a newer car. the old cars just vented into atmosphere, newer ones re burn excess hydrocarbon vapors. you can bring your cap to a garage/or new car dealer they have adapters to fit your cap, and they can pressurize it to see if it leaks.

I know my previous Saturn's cap hissed sometimes. I always thought it was odd that this one never made a sound. So, I guess it's not a big deal.