An overdrive tranny is the key to running fun gears on the street. But it does come with the price of adapting our A-bodies to an OD automatic.
Another key is the tranny choice.
Your 904 tranny gear ratios are 2.70/1.54/1.00.
A 727 has gear ratios of 2.45/1.45/1.00.
Your 904 also has less internal horsepower consumption than a 727 - 25 hp compared to 45 for a 727.
I've done a bunch of research and finally decided on going with a 200-4R behind my 360:
* Shorter and lighter than either the 904 or 727
* About the same parasitic horsepower loss as a 904
* Gear ratios are 2.74/1.57/1.00/0.67
The 0.67 OD is the real kicker in this deal. According to the Richmond Gear calculator (http://www.richmondgear.com/index.php/gear-transmission-calculators), I can be at the following RPM at 70 mph with a 28"-tall tire:
3.73 gear = 1949rpm
3.91 gear = 2043rpm
4.10 gear = 2143rpm
4.30 gear = 2247rpm
4.56 gear = 2383rpm
4.88 gear = 2550rpm
Years ago I ran a '69 Dart Swinger on the street with a stout 340, reverse-pattern manual shift 727 and 4.88 gears. It was fun from stoplight to stoplight, but I couldn't take it on the freeway.
Now with an OD trans, it will be a win/win for all - except that guy in the next lane over!
