Diesel vs Gas environmental impact
 

I don't want to start a debate about which is the overall cleaner fuel, because that is a huge question with many different factors involved. Suffice to say that diesel advocates often point to reduced CO2 when they suggest that diesel is ultimately the cleaner fuel. Much of their argument, I believe, is based on the inherently better fuel efficiency of the compression ignition engine.

My question concerns the btu content of the two fuels. Diesel has about 13-14% higher btu content per gallon, which is why comparing the efficiency of spark ignition vs compression ignition based on mpg is misleading. So is the claim that diesel is cleaner based on better fuel economy likewise exaggerated? If diesel has 13-14% more btus per gallon, does it emit that much more CO2 per gallon burned? If this is true, then a diesel vehicle would have to get 13-14% better fuel economy than a gasser in order just to have the same CO2 emissions. Or does diesel actually emit less CO2 per gallon (or even per btu)?

Also, how much energy and crude oil goes into producing the two fuels. Again, I know this is a complicated question because diesel is a by product of refining gasoline, so essentially you can't have one without the other I suppose, but which has the greater environmental impact in getting to the pump, if that can be measured?

Diesel does emit more CO2 per gallon burned, I forget how much, but it can be looked up easily.

I don't know about production though.

I think the increase in Carbon is proportional to the difference in energy content, but offload efficiency is much better for diesel drivetrains, so there's also an improvement in thermodynamic efficiency during low power use. For instance, a 05 diesel and gasoline drivetrain in the same car result in 24mpg and 35mpg respectively, with a correction for energy content. Over the new EPA cycle, the diesel is nearly one and a half times more efficient than the gasser, in the same car. The more efficient the car is, the more pronounced this difference can be. At high load, the difference in efficiency is minimal, so if someone's commute consists of 90% highway at 70mph, there won't be a huge difference between diesel and gas. Otoh, if they can cruise to work in top gear on 40mph roads, there will be a much larger difference, maybe even a relative doubling in mileage.

Ya really!

But there can be so many differences there besides fuel, like weight, gearing, power output, styling packages, blah blah blah!

Inverse respectively. :o

Not really. At least not for the comparison I used.

List the models, punk.

Same trim level, 115hp gasoline engine compared to 100hp diesel engine. Diesel powered car weighs 2989, gasoline powered car weighs 2743. Manual transmission ratios are
Gasoline-3.300 1.944 1.308 1.029 0.837 (3.938)
Diesel- 3.778 2.118 1.360 0.971 0.756 (3.389)

Jetta?