I just rechecked the reliability calculation and
@Sun_Killer's earlier statement is completely correct but I misunderstood the meaning

These seem to be the current numbers on tank division statistics:
Armor: Max Speed -0.12 km/h, Reliability -4.0%, +1.0 Amor per point
Reliability: Reliability +8.0%
Main Gun: Soft Attack +1.1, Max Speed -0.12, Reliability -4.0%, Hard Attack +0.6, Piercing +4.0
Engine: Max Speed +0.24 km/h
So an x-x-x-x tank does not suffer reliability loss and moves at the same speed as a the original model but has higher combat capabilities. The price of these type of models are:
1111: 50 experience
2222: 180 experience
3333: 390 experience
4444: 680 experience
5555: 1050 experience