3.91 good or bad

No problem.

The calculator says it's technically about a 265 RPM difference, which might not sound like much, but can make quite a difference.

In my 1972 Dart I currently have 3.91:1 with 235/60R-14 tires.
I plan on going to 3.55:1 and also with a slightly taller tire (245/60R-15) which will help that much more.

1:1 final drive + 3.91:1 gears + 25.1" tires (235/60R-14) = ~3,140 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 26.6" tires (245/60R-15) = ~2,690 RPM @ 60 MPH

That's a 450 RPM difference at 60 MPH!



If I went with the 3.73:1...

1:1 final drive + 3.73:1 gears + 26.6" tires (245/60R-15) = ~2,830 RPM @ 60 MPH

310 RPM difference from the current 3.91:1 set up
&
140 RPM more then if I went with the 3.55:1 combo.