In my quest to educate myself and make the best decision for my family, this is what I've come to understand. I'm no expert and this is not intended as a be all end all. I'm learning and I'm positive there are others as well. My hope is that this can become a tool to those who choose to venture to the 'dark side'.
One figured bandied about is 30%, as in Diesel can offer up to 30% gains in efficiency. If you compare the EcoDiesels 22 MPG combined figure with the HEMI's 17 MPG combined number we see a difference of 25.64%, the theory holds water. But WHY?
Well for one its the fuel itself, with 15% more energy density per gallon of diesel vs gasoline the fuel itself inherently carries more work potential.
Adding to the efficiencies are higher compression ratios, ostensibly the higher the compression ratio the more power is extracted from the fuel itself, which adds an extra punch of performance as well as maximizing efficiencies. Of course gains are not limited to simply the fuel itself.
Diesel engines themselves enjoy mechanical advantages over their gasoline propelled counterparts. Variable vane Turbo's are a major one, allowing the boost to modulate which in turn delivers torque in a more linear fashion than one big GOB. From what I understand diesels also lack a throttle plate which eliminate pumping loses gasoline cars suffer from.
As I said above, by no means is this a be all end all, but simply a jumping off point for discussion and education.