What are the trade offs going from 37" to 40"?
The biggest trade off going to 40's is that you WILL start breaking shit IF you actually play as hard as you think you do.
Doing 40s right (IMO) means full width axles and changes in back spacing along with other mods. Does the wider track width have down sides or is the extra 3" to 4" not noticeable?
Doing 40's "right" is relative to how hard you actually push your Jeep. As mentioned, a wider track will allow you to lower your COG but a wider width can be a liability on some trails. This is the biggest reason why I've worked on slimming my JK's down. That being said, the biggest reason to upgrade your axles is for STRENGTH - PERIOD.
I realize the extra unstrung weight and rolling mass are likely horsepower sapping but how much on a V8? Just barely noticeable and 5.13 to 5.38 fixes it or so much that horsepower mods are needed?
"Needed"? Far from it. You drive a Jeep, not a Corvette - it's a brick in the wind and that it can reach highway speeds at all is a wonder in itself. Is more HP "nice to have"? Well duh! :crazyeyes:
In spite of what some will suggest, I ran Moby on 40's for years with just a 3.8L and people still bitched at me for driving too fast.
Coilovers would likely allow one to keep the same ride height (4") they had with springs and shocks on 37s? Yes? No?
You should be able to keep whatever ride height like 4" you want with or without coil overs. Ride height is NOT why you'd want coil overs.
Has anyone on here gone to 40s, not liked it and gone back to 37s? If so, why?
As mentioned, I've run 40's and gone back down to 37's but not because I didn't like it but rather, it was stupid of me to do and use my Jeep the way I do being that I only have a ProRock 44 up front. I only ran that setup to test the Dynatrac Progrip brakes. Based on past experience running 40's on a 44 in the past, I know it wouldn't take long for me to break something.
Finally, what's the performance / capability advantage of stepping up to 40s? I've wheeled on 37s and 60s front and rear with the old TJ and loved the unsparing weight. It was like the TJ was stuck to the rocks. JKUs are much bigger so my thinking is 40s are to a JKU what 37s were to a TJ.
I've run a TJ with 37's and can tell you, the TJ is no JK and far from being a JKU on 40's. If you have to ask what the "performance gains" are, you don't play on the kind of trails or obstacles that would make them nice to have. This is NOT a put down but rather, a reality check based on my opinion.
2) differences in on road driving manners are
Note: mine is not a daily driver but I don't want a borderline trailer queen either.
In my opinion, you ain't building a Jeep up right if you didn't consider it to be just as comfortable or handle as well as a daily driver.