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?
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2008 EPA adjusted:
Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)
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.