How fast is Formula One? Top championships compared | 2015 F1 season

Which racing series is closest to F1 performance levels? And has F1's switch to V6 turbo engines allowed them to close the gap.

Source: http://feedproxy.google.com/~r/f1fanatic/~3/szDPG_7TxrU/

Kazuki Nakajima Satoru Nakajima Shinji Nakano Duke Nalon Alessandro Nannini