Air resistance affects the fuel consumption of a vehicle. What affects air resistance the most?

You answered correctly. Air resistance, also known as drag, increases significantly as the speed of a vehicle rises. This means that the faster you drive, the more air your vehicle has to push aside, which increases fuel consumption. Neither the number of passengers nor the profile depth of the tyres have a significant direct effect on air resistance. For future questions, remember that among various factors, vehicle speed is what most strongly influences air resistance and thus has the biggest impact on fuel use at higher speeds.