beefed d30 and 35s is about all I would do. 37s even with open diff is gonna go kaboom

 

Ring and pinion is really going to be you weak point on a 30, and especially if you are going with 5.13's. With that kind of ratio on a D30, the pinion is just tiny. Heck... even with a 4.88 it is small, but again, it's a 30.

The difference in diameter from 3.73's to 5.13's is about 72%, meaning that if the diameter of the stock pinion is 2", the diameter of the 5.13 pinion is 1.45". (just random numbers... I don't know the exact diameter) That is one point of common confusion in the JK world... while we are the first to jump on tall gears to compensate for our "minivan engine", we often forget that we build in a little bit of weakness by doing so.

That said, the quality of manufacturing for aftermarket gears seems to be better than stock anyway. (the only failures I personally have seen came from stock gears) If we even get a 25% strength improvement from better metals, it might well offset the difference in size. And... because our engines are known to be weaker, there is less chance of grenading the R&P.

This is a long winded way of saying... R&P are the biggest weakness in a D30 and there are not a lot of good ways to offset that. Axle shafts and u-joints are another weakness, but they are easy to replace and a relatively inexpensive fix. And of course... the whole problem with the tube and C's, but that is not a problem specific to the D30's and should be fixed if you plan on wheeling hard with a mopar 44 as well.

 

Not true, the D44 u-joints are a series larger than the D30.

http://project-jk.com/wp-gallery2.php?g2_itemId=79190

 

I was really frightened when I saw how small the pinion was on my 5.13 for my D30. It made me wonder if 4.88s would have been smarter. But I spend 99% of my time on the road, and offroad I am very conservitive on the stupid pedal so I am happy with my choice. That said, I am thinking about going D44 over building my D30 like I had planned. We will see I guess.

 

I didn't mean to scare you away from building up your 30... heck, I'm building up mine... but I was kind of trying to make a point that the tallest gears vendors produce for a certain axle are kind of "at the tipping point". That is why I opted to go with 4.88's... while the pinion is still smaller than a 3.73 or 4.10, it wasn't the last stop available.

Seriously, if you are just running it up and down the road you will never generate enough torque with the 3.8 to be concerned. I was more or less afraid that when I was out on the trail in 4Lo with 36" TSL's, that would push it over the edge. It is really a fine line to walk, and I already have a bad habit of using too much skinny pedal.

I will admit that building the 30 is probably going to be a 12 month solution for me. I just couldn't come up with the cash for a 60 right now, but I knew I needed to step up to the plate to be able to keep up with the big boys. Running the 36's with 3.73 is just miserable, and I expected to replace my street tires before the end of the year as well. So... the stars just lined up to build as much as I could afford.

I also know guys who have wheeled XJ's with a HP30/44 combo for a long time without problems, so maybe I am just over cautions. Of course, I saw G.I.JOE¤MFA¤'s 39.5" Pitt Bulls today and have to admit that all of a sudden my 36" tires seemed a little inferior.

 

Put as much frikkin tire on it you want. When it breaks, you'll know what it cannot take.