GM OD tranny fit?

Why all the hoopla in regards to which transmission when you are actually speaking of gear ratios and which ratio should one be running to achieve what it is that is wanted. Running a 518 transmission for gas mileage still has you running low ratio high number rear end ratios like 3:55 to 4:10 and we all know what these ring and pinions can cost which will still put us in the ball park we are comfortable with. Of course there is other things like figuring out tire height also which has to effect just about everything in order to figure out the complete game plan. All this plays a part in coming out of the chute at a decent pace and lastly what RPM will be at cruising the highways

518 transmission - 2.45 first gear, 1.45 second, 1 to 1 third, .69 OD

2004R transmission - 2.74 first gear, 1.57 second, 1 to 1 third, .67 OD

700R4 transmission - 3.06 first gear, 1.62 second, 1 to 1 third, .70 OD