My stock JK has DANA30(front)/DANA44(rear) axle.
the axle gear ratio is 3.73 (and I assume it is for both axles, correct me if I am wrong)

I am planning to do a mild lift plus bigger tire. The question is,
what is biggest tire size my stock axle can handle?

 

I heard a theory that tire should slip first before axle breaks.

So there must be a calculation/comparision method. Can anyone
give me the way to do this calculation?

 

depends completely on the driver and were you go. i've seen broken axles with stock tires and nothing brake with 37" tires in the same hole. to be safe keep it under 35" without any added support.

 

As a general rule you need a D60 to run 37s, a D44 to run 35s, and a D30 to run 33s. Many on the forum run 35s on a D30 with no problems so far.

So if you want to play it safe run 33s, and if you want to play the odds go 35s and you will probably be OK, but I would not recomend 37s on a D30.

 

I broke my D44 front shafts with stock tires last winter!

 

Really depends on your use of the skinny pedal, and how much you let your rig hop. I ran 33s on a 35 with no issues at all, but I'm not a big fan of the skinny pedal and the minute I start to hop I hit the clutch pedal.

 

Depends on what you plan on doing with your rig... for light wheelin the 35" tire should be fine. If you get some c gussets they are cheap peace of mind. I would wait on axle shafts until one breaks because I am cheap...

 

I have the same axles as you and I'm doing some mild wheeling with 33"s. I plan on going with 35"s when I get the money but I'm also going to upgrade my axle shafts and gusset the axle aswell. But I've seen guys running 35"s with no problems at all but just need to change out for the gearing.