I don't think this is correct. The heat engine (gasoline or diesel) can be made to handle the average power requirement at some speed, and the electric motor/generator is used to balance uphill and downhill grades. In a conventional car, the heat engine must be sized to handle the largest instantaneous power requirement, which is typically a steep uphill segment on a high speed road.

My wife and I drove our Honda Insight hybrid round trip from Colorado Springs to Amarillo recently (360 miles each way, 580 km) and averaged 69 miles per gallon, which is about 3,4 l/100 km or about 78 gCO2/km. We maintained, at her insistence, the speed limit (mostly 70 mph, 113 kph) and ran the air conditioning--and it was about 90 degrees (32 C).

By reducing the speed we could have achieved 70 g/km in this 1999-technology car, with no loss of comfort.