20l per 100 around town is pretty good. The best RB30 can clock is 6km/L as oppose 5km/L of TB42, but your figures above says 100km/13L, which translates to pretty much 6km/L. I see no difference comparing to RB30.
To me, it looks like 3L RB30 engine isn't producing any economical advantage over a TB42 when the TB42 is fine tuned and well maintained.