I don't know if 5.13 is enough for 40s even though that was my first thought. I'm running 4.88 for 37s and for daily driving it feels ok. I figured more like 5.38s but I have little experience with tires that large and auto trannys. That's why I'm putting the question out. I figure members like MaxPF have way more experience with these things.
"Ex Umbris Venimus"
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The biggest thing is to figure out your cruising RPMs and then get as close as you can with the new setup. That is, unless you feel you need more gear at highway speeds, but keep in mind that this will almost always exact a fuel economy penalty.
The gear calculator tells me that with 40's, 5.13's and 5.38's both bracket your current setup. Since bigger tires are heavier and have more rotating inertia, going with the deeper gears is usually a good bet. So, my choice would be 5.38's.
Here is an excellent gear calculator:
:link: Go to that link, and on both sides poke in 4L60E for the trans. Not sure what t-case you have, but I used the BW 4401 for your example. If you have the 4:1 case, then choose the NVG241OR, since they don't list the 4:1 BW case. Or, choose "Enter your own number" and enter whatever high and low ratios you want. In this case, choosing a proper t-case isn't a big deal since you are more interested in road RPMs than crawl ratio, but its always nice to see what your crawl ratio ends up being. Then put 4.88's and 37's on the left side, and 5.38's and 40's on the right, hit calculate, and see what comes up. I scroll to the bottom, which shows road speed in a given gear, and enter "55" "65" and "75" for the speeds of interest on both sides, and hit calculate again. You can enter different diff gears and re-calculate to your heart's content. I guarantee you will waste at least an hour messing around with that calculator
