I have 3:90 gears with the SM420 and have no problem cruising at 65. Its just a bit loud at that speed and 4th likes 55-60 best. I added a Gear Vendors O.D. which gives me the best of both worlds. I can go 80 -85 now in 4th over. 70 -75 is quiet and smooth.

1962 Fleetside 4spd 235 w/O.D. posi 3:90
In the Stovebolt Gallery
1990 Subaru Legacy Wagon AWD 2.2 5sp
2013 Ram Tradesman C/V
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