In my experience it's a three mph change between gears, mine was 10 mph I just kept buying them until I got it right, I should have just bought the whole set.
I ended up buying a no-name brand eBay kit years ago for my larger tires and 3.55 gears. I got the kit because I wasn't sure which gear I needed for sure and it was only a little more than the single gears anyway. I'm glad I did!
I need to change gears to fix the speedo reading, too. In the past, they were color coded (for number of teeth), do not know if they are anymore.
They still are, but also best to count teeth as whatever you buy is usually aftermarket now. Maybe entirely.
One thing i've learned about these trucks is the speedometer, and Odometer can give totally different readings. MY speedo says I'm doing 80 MPH while my phone GPS says I'm going 60 MPH and my Odometer is counting like I'm driving 55 MPH. You might change the speedo to the correct speed but what does the Odometer have to say about that?
That's nuts! I've never thought about that ever...MAYBE I'm not really getting 13-14mpgs after all!? I'll have to check sometime, thanks for the tip! I don't care about the total miles driven as it's a rollover anyway, but having accurate MPG's is something I always like to know.
And ideas how can the speedo be different than the odo?