[Big Dave]

Gas and diesel engines are mature technologies. You could throw a fairly small spread of overall efficiency over the whole market. If one were noticably poorer, they'd never sell it at half of today's fuel prices. Conversely if one got noticably better MPG, people would snap them up - just as they snap up VW TDI cars.

My diesel truck, weighing 7,000 lb gets 25 MPG on average right now. My 4,500 lb Impala SS gets 19-20. Same roads. Same driver. Same amount of hypermiling. The truck has a larger frontal area and a Cd maybe a little south of 0.40. The car has a smaller frontal area and a Cd of 0.34.

The car should get better MPG, but it doesn't get close.

The diesel/manual combo washes out the car's advantage of frontal area, Cd, and weight. That difference must be powerful.