I always hear it but I never understand why 3.73s are better for the automatic tranny than 4.10s. Why is that so?

 

I think that's a bunch of rubbish... I can see for a supercharger how it would probably be better. I don't see just because it's an auto. N/A Auto I would assume it would be pretty bitchin to have 4.10's with an auto shifter. It would tear through the gears. Maybe the possibility of smoking the tires? That would be just about the same for manuals if not even more so. However, that's taking the experienced pro driver out of the seat and putting in your averageJoe Schmoe.

 

The gearing in the trannys for an auto and manny are different. The 3.73 compliments the gearing in autos better. Also they are 3.31 to start with where manuals are often 3.55 to start with so bumping them to 3.73 is useless on a n/a car

 

I have 4.10s in my S/C auto. Insane wheel spin in first and second on stock rubber. Mine is not my daily driver and is wearing drag radials. Now it is a bullet.

 

0.675 OD (Manual) vs 0.71 OD (Auto).

Most of the dedicated performance guys can live with 4.10's in the manual. If your on the hardcore end of the spectrum 4.30's.
4.10's in an auto give ever so slightly higher top gear rpm than 4.30's in a manual.

3.90's are really the best compromise for street/strip but this puts you in Motive instead of Ford manufactured gears and many are afraid of the possibility of gear nose.

For your ultra-hardcore folks 4.30's + 27" ET Streets work well enough to run 11.89 @ 114.75 in a Full Bolt-On Auto Car.
That's Ron's (Spydershaft) car. Im thinking it traps around 6400 rpm in 4th.

Me personally I would have 4.10's and 28" Tall Tires on an Auto. With a manual Im going 4.10 + 26".