T-34/76 and (maybe) M4A3(75) v.s. Panzer
Tiger vs Sherman firefly and T-34/85
M4A3(76) vs T-34/85
The only tanks the M4A3(75) has any chance against in a head on engagment is a panzer or another M4A3(75). both T-34's have no possible frontal shots, and neither does the tiger.
The M4A3(76) and T-34/85 are pretty evenly matched, the 85mm looses its punch pretty fast, and up close, the 76 can kill the T-34/85.
Karnack has it close, but not quite right. On the panzer, the M4's 75 tends to give a lot of turreting shots at long range, and gives a lot of kills out to 1600yds. And the T-34/76 over the M4A3(75) because of its speed, turret traverse speed, and HVAP rounds.
The same goes to the T-34/85 up close, the turret travers and HVAP rounds will give it an edge, but less than the T-34/76's since the M1 AT gun can penetrate the T-34/85's armor up close.
I spend a lot of time in GV's, and have even tested them with friends in the DA. Those seem about the most even match ups you are going to get untill they add more tanks.
Hope that helps sir <S>,
Nemisis