Is there a difference in gas mileage between standard cab trucks and king cab trucks?

Lets take the 2.3L Ford Ranger for example since it's known to get the best mileage out of any sized pickup in the US. If one truck was just a standard cab and the other was a king cab, would the king cab do worse on gas since there's more room in the cab? I mean, the extra space really wouldn't add any pull or weight to the engine, right?|||It takes more metal to build a king cap than a standard cap. Which equals more weight,which means poorer gas mileage.