I'll toss this out since the topic kind of went in this direction-
EVERYTHING is a tool. What matters most is what is the most efficient tool to use for a particular job.
Regarding moving people one needs to consider People Miles Per Gallon (not just MPG). If Hummers were loaded to full occupant capacity they'd be getting more PMPG than a single-occupant Prius or TDi: it's extremely rare that one sees a fully loaded Hummer, but that is not the fault of the Hummer (though I don't like them), it's the fault of the owners/operators.
And, of course, when it comes to hauling stuff it matters how much mass you can haul efficiently- Mass Miles Per Gallon? I just used my farm truck to haul a trailer to pick up another trailer. Of all my "tools" this was the most efficient for the job (and likely more efficient than anything that I could have rented).
I got my TDIs because the wife and I have significant commutes: over 100 miles per day combined; about 50k miles last year (first year of having the TDIs). My car is currently settling in around 52 mpg; hers is about 50mpg. So, 53 PMPG and 50 PMPG; if we had an additional occupant then that would be: 106 PMPG and 100 PMPG.
Ryuyasha5004, I question your comment about the Prius: "uses almost double the amount of co2" etc.. The environmental concern, such that I understand it, is one of generating co2. Concerns with diesels is over NOX. (I'll keep from commenting on the global disaster that has been unleashed by the Prius builders' country; and, I'll keep from commenting on the US's number one environmentally "concerned" state -CA- and their Aliso Canyon gas leak - both of these likely far surpass the damage done by VAG's emission cheating.)
Back on topic... Mufflerectomy has no negative environmental impacts that I am aware of (db increase is negligible- I measured 2 db); if anything, not installing another new muffler means less new material mined and processed; and there's the minuscule (we know it's there, but likely too small to measure) gain in MPG due to weight loss.
