I Drill

I've got a 3" teraflex lift on a rubi unlimited with auto trany and 35x12.50's. I'm planning on regearing asap, and I know many of you will recommend 5.13's, However the other day I was talking to a guy at the stealership and he recommended 4.88's. He said 5.13's are over kill, and will decrease mpg, and cause other parts (like the pinion) to break in high torque off road situations. He also said the top speed would be about 65mph.

Your thoughts?

I Drill

I know alot of you are running 5.13's what do you think? is it too much?

tkob1060

that mechanic is talking out his ass. 5.13's are perfect. I would bet 95% of the guys here that have changed gears on autos and 35's have gone with 5.13's. Have you seen anyone complain they broke a pinion?

I can cruise at 80mph no problem with my 5.13's have even done 90 but only briefly, so um top speed is like 65 huh

I would NOTTTTTTTTTTT let a dealer do gear install, you want someone experienced at doing them.

I Drill

thanks for the feedback. I don't make a habbit of listening to the dealer, but this guy acted like he knew what he was talking about.

sandiasnow

I went with the 5.13's I love them!! So much better off road and on the freeway! I would say go with the 5.13's and you will be very happy with the why your jeep runs.

Rubidonn

5.13's and love em... and they improved my gas mileage a bit w/the 35's... around town, I'm back to around 15mpg or slightly over.

j33per

4.88's on my JK Rubi Auto with 35" tires. The JK with 3.8L engine needs all the help it can get so perhaps 5.13's would have been a better option!

I Drill

I'm feeling good about the 5.13's. Thanks for the feedback. For those running 5.13's did you reprogram with the Hypertech programmer?

Rubidonn

I haven't done that yet... I think with my 35's I'm about 5-6% off.

TEEJ

Well, Rubi's are calibrated for 32" tires...and 4.1 gear ratios.

I's about an 8.6% difference going from 32's to 35's...LOWERING your RPM at any given speed by about 8.6%.

The change from 4.1's to 5.13's is ~ 20%...RAISING your RPM by ~ 20% at any given speed...


So the end result is that with 35's LOWERING your rpm, and the 5.13's RAISING your rpm, the end result is essentially to take your CURRENT rpm at any given speed, and subtract 8.6%, and then add 20% to the new rpm, to get the final change.

IE:

2,000 RPM MINUS 8.6% = 1,828 RPM (Going from 32's to 35's lowered your RPM by 172 rpm...THAT's what you are compensating for...loosing 172 RPM...)

1,828 RPM PLUS 20% = ~ 2,194 RPM

2,194 RPM is ~ 8.8 %



So - Overall - the 35's and 5.13's end up with you running at ~ 9% HIGHER RPM than when you had 32's and 4.1's.




(For comparison, the 4.88's raise your rpm to ~ 16% more than 4.1's...or to 2,120 rpm from 2,000 rpm, a roughly 4% difference.... from the 20% that the 5.13's raised it, etc...)

Essentially, you'd be hard pressed to tell the difference between 2,120 and 2,194 RPM anyway by the seat of your pants.



The higher the ratio, the more teeth are needed....in the same surface...so they have to be smaller to fit.

That means the teeth on the 5.13's are going to be smaller than the teeth on the 4.88's - and that the teeth on the 4.88's are going to be smaller than the teeth on the 4.1's....etc.

Generally, the smaller the teeth, the more easily they can break.

I do NOT think the tooth shrinkage from 4.88 to 5.13 is that big a deal though.



I also love the lower gears (Higher ratio) off road...but, frankly, mpg is really better at LOWER, not higher, rpm....and, if you need to downshift - well, its free.

All the downshift does is temporarily replace the gearing, so your final drive ratio is the same as if you'd swapped in lower diff gears, etc.



People with autos vs manuals will notice that you can shift the manual tranny with 6 speeds...so the autos are less flexible in the rpm department, etc.

So, those with manuals will report a different experience/happiness with their gearing than people with automagic trannies.