okay to keep your larger tires and have a reasonable speedometer reading,
you need about a 13-5 transmission ratio gear set
the bad news is that there isn't one listed that is compatible with a sm420
the few gear sets of that ratio are for the automatic, the 3 speed and very old 4 speeds.
perhaps you need to find an adapter to bump up the speedometer reading by about 10% because your wheels get bigger and your truck is moving faster than normally indicated.
if you stick with the factory ratio in the transmission (hunt down the 3709359 part, 7-20 teeth your truck is supposed to have) then find 1565812 speedometer adapter
do not wast time on the other similar part number adapters, they are very different.
1565814 is used when you make the wheels smaller, and decreases speedometer indication by about 3% based on what i found, this is the wrong direction for you.
1565813 is used when you make the wheels larger and it increases speedometer indication by 1:1.133 however that's a bit too much, this one is listed in parts manuals when the wheels change by even 15%
1565812 however is used for about just your use case
- when going from 700-17 6/8ply to 750-17 8 ply ~9% change
- when going from 650-20 6/8ply to 750-20 8/10ply ~9% change
best of luck
-s
