Interesting comparisons Peter, The DA42 TDI does a little under 9 galls per hour (total) at about 150kts at FL100. Even with the UK tax rules it still works out at approximatly 65% of the fuel cost of Avgas, so a saving per mile of around 35%. Even more saving if its a training flight.
There is of course the added safety advantadge of a proper deice system, two engines and a significant improvement in instrumentation / autopilot system.
The other advantadge is a composite construction with modern aerodynamics and wing section, less parasitic drag, higher aerodynamic efficency etc..
Whilst the TB20 is a great aeroplane, natural evolution has seen its design superseded.
Will they do a diesel version equivalent of the IO540?