Interesting... Green Car Congress: ORNL researchers quantify the effect of increasing highway speed on fuel economy - Spindifferent
Our 1.8L Prius and the 1.5L Prius c engines use cooled exhaust to avoid high-power enrichment: Source: ORNL report Bob Wilson
Are you talking about the exhaust heat recovery system? I thought it only circulates coolant to the exhaust so the coolant will warm up faster, and it shuts the valves off then the coolant is at temp.