Looking up the RPMs from the other thread reminded me to do this.
So here are some key RPMs and speeds for the factory 2.93s that my Roadmaster has (oops, I did this with 225/75-15 wagon tires, redoing with my 255/50-17s):
5000 rpm in 1st is around 44 mph (with the torque converter locked up so in the real world the speed would be a bit lower).
5000 rpm in 2nd is around 83 mph.
5000 rpm in 3rd is 135. Not terrible top speed gearing but if it had the 225/75-15 wagon tires (which aren't going to be speed rated) it could pull 145 in 3rd which is about right. Well, with RMS aero, maybe 140 is more realistic but with torque converter stall that could still work out closely.
And then here are some RPMs and MPHs in 4th lockup:
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RPM MPH
1043 40
1173 45
1303 50
1434 55
1564 60
1694 65
1825 70
1955 75
2086 80
2216 85
2346 90
2477 95
2607 100
I think most people would say that gearing is still fairly tall. And I'd agree. Of course 2.56 is 14.4% worse across the board, which is VERY noticeable by seat the pants dyno.
So shifting to the 3.42 buys about 16.7% extra RPMs everywhere, so BIGGER than the already noticeable difference between 2.93 and 2.56. Good so far. Let's compare to above:
5000 rpm in 1st is now around 38 mph. Pretty good. A little higher shift RPM would extend it a couple MPH.
5000 rpm in 2nd is around 71 mph. With more aggressive shift timing, this would really help with the 30-70 acceleration but over 65-70 it's going to stay in 3rd, which is only like 2800-3000 rpm. Previously the 2.93s helped a little up there because they carried you above 80.
5000 rpm in 3rd is now only 115 mph which is still more than necessary to run right into the 108 limiter. If I get rid of that, it will need to do a WOT 3-4. But this is definitely looking a lot more like performance car shift points (actual shift points are probably a bit different but this is ballpark).
4th lockup will look like this with 3.42s:
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RPM MPH
1217 40
1369 45
1521 50
1674 55
1826 60
1978 65
2130 70
2282 75
2434 80
2586 85
2739 90
2891 95
3043 100
Definitely better cruising RPMs for any attempt at acceleration or maintaining speed on hills instead of coming out of lockup or downshifting.
Seems like a totally reasonable gear. After all, that was the factory ratio in 89-93 Caprice 9C1s (and 94-96 Fleetwoods with V4P towing, many Corvettes, etc.). Point is, to the extent that a factory 2.93 car drives WAY better (even with the mechanical fan) than a 2.56 car, the 3.42 is actually slightly MORE than that difference. Sounds great to me. It will, as they say, "really wake the car up."