The M3 is an inferior car, why even consider it?
First of all, all ratios are relative to one wheel turn. For example, a ratio of 3.14:1 means that the engine has to turn 3.14 times to turn the wheel one time. The final drive ratio multiplied by the gear ratio gives you the final ratio. So for example, if your fifth gear has a ratio of .85:1, then the final ratio will be
0.85 x 3.14 = 2.67
Then you figure your tire circumference, how fast you want to go, and figure out how many rpms it would take to get to that speed. Then you look at the torque curves for the vehicle and see if your gear ratios fit properly for what you need the car to do. For example, you don't want to shift to 3rd gear at 55mph if you're looking for good 0-60 times. Then again, some cars, like Mercedes' are engineered to have enough torque to pass safely at 80-100mph. Those comparisons between the M3 and '55 don't take into account the type of car and engineering philosophy behind each car. I have never really understood the significance of 0-60 times. If you wanna push it, an electric car will outperform a Ferrari in a 0-60 test. Why not 0-80, or 0-100? 1/4 mile figures make a lot more sense to me.
Anyway, to get back on topic, it's not just because of the final drive ratio, it's the ratio of the gears from 1-6 and the point at which shifting occurs, and where the engine makes the most torque, and how the gear ratios are optimized to make full use of this torque.