All kinds of things effect fuel economy. Do you carry a lot of extra weight? Stop and go versus uncongested freeway, cruising speed (80 mph versus 70 mph), unnecessary braking (tailgating), coasting to an upcoming stop versus heavy braking, tire type, size, and weight, fuel quality (contains ethanol or not), driving surface (gravel, concrete, asphalt), etc. Any of these conditions could make a significant difference in your vehicles fuel consumption. Speed could make a difference in excess of 10%.