Right some how the over all cost of going to T3 dropping down by 20 fuel makes going Penals first and the T2 which costs 15 fuel significantly slow down your T70.
I'm sorry the math doesn't add up.
You're buying a ZIS gun. That costs 320 manpower. Also, you're buying T2, which costs 20 fuel. Do I really have to explain it?
Even if you just buy T2 (why would you do that if you're not even going to get the zis?) the T70 is coming out slower than if you didn't. The slower you get it out, the less time it has in its window to be the most impactful on the game.