Of course, you won't see any difference on the speedo, since it measures axle rotation and assumes a tire size. You can correct in your head based on the numbers derived from the site above, or use GPS, or you can re-calibrate the PSOM (can only be done a certain number of times).
Here is an awesome gearing calculator. It is set up on the page twice, so you can input your stock stuff on the left, set up anything you want on the right, and then compare the two.
Using that calculator i see you turning 2158 @ 70 mph. To get 200 rpm lower, it says you need a tire that measures 35" tall, which means you need 36s or 37s to get what youre looking for. Take a look at that link, do some calculations, and you can decide what you think will be the best option for your truck
madpogue is right; if you change tire size the speedo wont be accurate unless you re-calibrate it for the bigger tires.
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