I have a 3.8 auto Unlimited with 33's. I want to regear to 5.13's, but I'm concerned about possible downshifts. For example, sometimes my JK will downshift to 2nd at 65 just to accelerate to 70, then go to 3rd then OD. If I had 5.13's, my calculations show a downshift to 2nd would put it way over redline. If that downshift occurred, would gas be shut off from the engine to prevent damage?

 

4.88s or 4.56s will be better. If you plan to move to 35s then 5.13s would work

 

I think you'd be way over geared for 33s. Unless you plan on 35s +, I'd stay away from 5:13s.

 

I ran 5.13's with 33's for about 6 months before getting 35's. You'll be fine.

The numbers from the JK charts...

3.8 auto with 33's (that measure out to 32") :
OD On = 2603 rpm @ 70

OD is .69, so divide 2603 by .69 to get the OD Off rpm.
OD Off = 3772 rpm @ 70

That is a fairly typical rpm for me on the hills/mountains around here. Not really unusual for me to be over 4k.

If you are worried about over-revving, here's a hint -- let off the gas ...

 

Thank you everyone. I am planning on 35's eventually so 5.13 should work well.

 

I have run 5.13's on my 2010 Rubicon JKU since new with true 33's and find it just about ideal, having said that I live in the mountains in western NC, but do drive it on occasion on the interstate to Atlanta at 65-70 mph with out excessive rpm....just sayin.

 

Don't forget to get a programmer. It will optimize the shift points and keep the Jeep from throwing on the stability crap when you corner hard.