Let me throw in another opinion. A long-time rule of thumb that has always worked for me, is that to get the best torque multiplication in 1st gear, the 1st gear ratio times the final drive ratio should be about 12. Example: a 2.95 1st gear would perform best with a rear end ratio of 3.90 or 4.11. This was about the mix used on early six-cylinder pickups and passenger cars. There are exceptions of course - Power-to-weight ratio can change this. Musclecars can get away with a 2.20 1st gear because of the brute torque of a big block motor. Much lower and the tires will go up in smoke.
Note that this 'formula' does not consider factors like RPM at cruising speed, fuel mileage etc. It is strictly concerned with gearing for getting a manual-transmission vehicle moving from a standing start, in normal driving.
I have absolutely no documentation for this rule-of-thumb. Where it originated, I have no idea, but I have heard it mentioned by several old-timers and it has always produced good results for me.
