Re : 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jetta TDI I like playing with numbers and I would like to to discuss those BSFC numbers a little further. Sorry for the metric units - The TDI is supposedly achieving peak efficiency of 190 g/kWh so 1 kg of fuel gives you 5.2 kWh and 1 liter of diesel (0.840 kg/lit) gives you 4.368 kWh - Atkinsonized Accord petrol engine gives you 214 g/kWh or 4.6 kWh/kg or only 3.5 kWh/l (0.750 kg/l) The difference per volume of fuel is HUGE on paper, diesel engine is supposed to get 25% more energy out the same volume of fuel, therefore a diesel engine should get you 25% more MPG on cc on the highway than a hybrid because of the engine alone. Then you have drive train loses that are bigger on the hybrid because of energy transferred to generator and el. motor than a normal TDI with the stick, we are talking about another 3-5% easily. Then I look at the Accord and Passat TDI speed vs fuel economy and Accord is better at the 50 MPH and at 70 mph Accord is only 12% behind in MPG. Where did that 28+% boost of the TDI MPG go? I can tell you: The 2.0 TDI is achieving it's peak efficiency at higher load, I would guess that around 50 kW (250Nm @ 2,000 rpm), that kind of power will get you 100 MPH on the straight easily, but at that speed engine on a MT will not rev at 2,000 rpm. Atkinson petrol engine on the other hand will have it's sweet spot considerably lower (12-30 kW?) and you are using that power a lot more than 50 kW. Even when you are hypermiling I seriously doubt that you are getting in this high region, it's just not efficient driving, or am I wrong? Under lighter load a hybrid petrol engine must be at least 5% more efficient than diesel engine to achieve those final numbers at 60 MPH, because of less energy per volume, remember BSFC number is per mass not volume.