For the TB20, I see they got the wrong engine; it's an IO540 not a TIO-540 which the TB21 has (but the TB21 is not a -C4D5D).
The MPG data (which is perhaps the main meaningful comparison between planes and cars) is awfully hard to summarise in this way, because nowadays almost anybody with a brain flies LOP and not at 75%, and the MPG goes way up. So the "real" performance data is not found in the generic flight manual (which is where this stuff was extracted from, presumably).
For the TB20 the table lists 11.5 NM/USG but the real figure (at max available MP, FL100-180, LOP) is about 16. Consequently the range they give (986nm) is more like 1300+ nm.
16 US MPG is about 22 UK MPG (hope I got that right) which is damn close to what my 3 litre Toyota import does, and its max HP is.... 250 which is the same as the TB20! Its empty weight is 1800kg whereas the TB20 is 1400kg (similar).
Physics is physics; no free lunch to be had anywhere.
If you want to have some real fun, work out the fuel burn per passenger kilometre for say the TB20 (or any common GA type, operated sensibly) and a 737 / 747 / 777 / 787 / A380 etc. A lot less difference than you might expect. Most of them burn the same fuel from LGW to Orlando as you would burn in a Ford Escort driving from LGW to Orlando.
No free lunch.