12.7 = every 12.7 engine turns, rear wheel will turn once for manual gear. This means more engine torque is available for acceleration.
12.4 = every 12.4 engine turns, rear wheel will turn once for automatic gear. This means although torque available for acceleration is slightly less than manual gear, you will reach slightly higher speed before changing to 2nd gear.
If you want to calculate vehicle speed,
Say, at maximum 1st gear speed or engine RPM of 6500.
Engine speed is 6500.
Through gear box, you have 6500/4.35 = 1494 RPM
Put 1494 RPM through final drive ratio = 1494 / 2.93 = 510 RPM at wheel or 8.5 tire rotations per second (makes sense). If you want to do this quickly, 6500/12.7455 = 510 RPM.
Now, 18" wheel with 255 wide 35 profile will give you 18x25.4 + 255x0.35x2 = wheel diameter of 457mm+179mm=636mm or 0.636m
Rotate 0.636 m diameter wheel 510 times per minute or 510x60=30600 times per hour = 0.636x3.14x30600=61109m/hour or 61 km/hr. Fun, eh?
Now take the torque curve of the engine, put it through gear, calculate the force pushing on the car (taking into account rotational inertia of the wheels, suspension jounce, frictional losses, aerodynamic losses, etc) and you can estimate 0-60mph time!
I'm sure you could put the formulae into Excel sheet and do it much faster . . .