The difference in horsepower at 70mph between 4.88s and 5.13s is about 5hp. While that sounds somewhat nominal, on a vehicle that isn't putting that much power to the ground it's a lot. In round numbers, every additional 100rpm gives you 4 more hp at the wheels. The torque curve on the mighty 3.8 doesn't start to flatten out until you reach 2500 rpm so there isn't any diminishing returns up until that point.

 

The rules for gearing a car for drag racing are much different than gearing a JK for on or off road. With drag racing we generally have an abundance of power. We also look to changing out the torque converter to flash us into the power band more quickly. So the rules quickly become under gear and over convert for drag racing.

With the JK we have a low stall speed converter which practically no one changes. Then we add on top of that the under powered engine. What this means is that we need to make power more quickly and we need to start that process at low rpm rates. The way we get there is by changing gear ratios so we climb the rpm range quicker, getting into the power band sooner.

 

Go to 5.13. If you want gas mileage, get a Honda.

 

Looks like most of you guys are going with 5:13's. I'll leave the gas mileage to my other half as she does drive a honda.

 

I rather have the added ummpphhh from the regear to 5.13s and make the Jeep actually fun to drive rather then worry about getting better gas mileage any day

 

I hope that I see 15mpg when I install my 5.13s, right now I an getting 12.2 mpg with an auto trans, 35s and 4.10s

 

From the HP curve chart I have seen, 2380 RPM is about 82 HP, 1902 RPM is about 60 HP, so at these engine speeds going from 4.10s to 5.13s gives you a 34% increase in HP.

 

5.13s can be harder on the d30 and lead to failure faster I think...from what I have seen on here anyway, it seems that way. 5.13s with lockers in a d30 are pushing it from what I understand. Yes/no?

 

People have been saying that for quite a while now, but very few are actually experiencing strength issues with the 5.13's. There was one post not too long ago with some great pics of a D30 with shattered alloy axle shafts, (and I believe a twisted drive shaft as well) and the 5.13 R&P was fine.

If you want the performance of 5.13's, get 5.13's. Odds are, you'll toast the driveshaft or axle shafts before the gears...

 

It Boggles me as well. The week I had 4.10s and 35s was a lugging, moping, crawling sad time. Ive seen guys say that 3.73s and 35s are not bad???? I say wha???? Maybe 35's and 4.10's = 1902rpm @ 70mph but I wouldn't know cause my tranny was searching for the right gear the whole time at 70mph (that is until I got fed up and turned OD off or dropped speed to 62mph)