I find that if I keep the thing around 90-95km/h (when towing) we get the best economy. I'd get better if I could keep her between 80-85km/h because of the reduced revs, but at that speed there's not enough torque to keep her going. At 95km/h she's sitting bang on the top of the torque curve's main climb, resulting in the most torque available for the least amount of fuel. It does develop more torque at higher RPM, but the fuel flow rates are substantially higher.