magilla97

I was at the OK Auto 4x4 open house today and asking a bunch of questions. I met a couple of techs from AEV and asked them what they run for gears in their 4dr auto JKs. They told me that running 4.88 with the auto was the way to go, regardless of the engine, for 35" tires. So now I am confused once again and given the expense of regearing I don't want to get it wrong. I will be doing a fair amount of highway driving and my main concern with the 5.13s is that the RPMs will be too high at 70/75mph. Can anyone with 4.88 or 5.13 autos provide some insight? If you have specifics on the RPMs you're running at these speeds that would help. I don't want my Jeep screaming at 3000 RPMs at 70/75. I have looked at numerous threads here and elsewhere that point to 5.13, but the advice of the AEV and techs at OK Auto has me questioning whether 4.88 would be more appropriate. Thanks.

tkob1060

As much as the Aev guy knows, I have to disagree totally, I have 513's and 35s, driven 40k mi on them and they are perfect, I run about 2500-2700 rpm at 70-75mph

not screaming at all and right in the rpm range the 3.8 likes to be in, I also have no problem passing on highway, I think I would even like 5:38s....actually another member recently got 5.38's on his auto recommended by teraflex and he loves them, he does live in Ut or CO so a bit more mountains.

tkob1060

May 30th a bunch of us are going to Pine Barrons, come out and wheel with us and I'll let you drive mine down the highway to see how 5:13's
https://www.jk-forum.com/forums/east-coast-51/wharton-trail-ride-sat-may-30th-80675/

was he running a hemi?

davidwm71

With stock engine or the AEV Hemi upgrade. Is that what is meant by regardless of engine?

magilla97

The tech was telling me that they would recommend 4.88 with both the stock 3.8 and the Hemi. This opinion was shared by another tech from the shop hosting the event. I honestly didn't think the 4.10 was that bad until I drove up to Rausch Creek today and was struggling up 501 on the way in and out. I had to kill OD and still was plodding my way up.

chuck45

As much as I like the AEV guys they are way off base on this. It is ludicrous on the face of it to claim that one should run the same gears with a 3.8 as a 5.7. If I had a 5.7, an auto and 35's I imagine 4.88's would be just fine.

I still have the 3.8, have an auto, 35's and 5.13's and run 2600 rpm down the freeway at 75 mph (by GPS). 2500-2600 is the minimum rpm you want to be at that speed and in all honesty I'd rather be at 2700-2750. The real question is not whether to run 4.88 or 5.13 but 5.13 or 5.38's. If you're a heavy 4 dr you would be better of with the 5.38's. I wouldn't waste my money on 4.88's; that is unless I had a stick.

Finnster

I'm not a gear expert by any means but, if a stock Rubi with an auto runs at about 2200-2300 rpms at 75mph then wouldn't you want to regear to replicate this scenario? I think the engineers must have considered the characteristics of the engine/torque curve to come to this conclusion. This is why different JK models have different gearings to compensate for this. I think if you do the "math" 4.88's with 35's come out to about the same end result with regards to gearing.

Now with that being said there are other factors that come into play such as more friction and more unsprung weight by running 35's.

Now....maybe EPA/Gov't played a role in gearing the stock Jeeps to keep MPG's up

RedneckJeep

Ask a thousend people how to do something and you're likely to get as many different responses. Do yourself a favor. Research it for YOURSELF, instead of asking everybody else. Asking everybody else will net you only one thing. How THEY would do something. The more you investigate and research, the more you'll be empowered to make the decision that best fits YOUR needs and not somebody else's.

chuck45

So you're going to leave your JK stock because that is the way Jeep engineers designed it and they know best right? No - I didn't think so. Having had my Rubi stock for 2000 miles before I started modifying it I feel very comfortable saying that the stock Rubi is geared WRONG WRONG WRONG. It was very had to keep it in OD on the freeway unless you were dead level or going downhill and the cruise was useless. The JK is a heavy brick and takes power to move it thru the air at speed and 2250 rpm is too low in the torque curve. With my 35's and 5.13's I'm at 2600 rpm, have much better power, stay in OD much better, canuse the cruise, and get exactly the same fuel milage. And this is from a vehicle that is much heavier, lifted and with bigger rubber. In fact if I go to D60's and keep the 3.8 I'd definetly go to 5.38 gearing so that my rpm would be up where I think it should really be; between 2700 and 2800 rpm.

Why you would want to "do the "math" 4.88's with 35's come out to about the same end result with regards to gearing" is beyond me. Stock is lousy.

If you can get to the Moab EJS and sit in on a seminar with Jeep engineers the answer to damn near every question or complaint is "governtment regulations". They have constraints that we don't. They have to satify the government and bean counters - we don't (well except for the better half).

Finnster

Well, I never said I was going to leave mine stock I was merely just trying to figure out why AEV would say that. I already have my 5:13's in my garage, just waiting on the rest of the project list to show up

As RedNeckJeep said, do your homework/research. I have done this for a bit now and 5:13's seem to be the "best" for me given my region of the country.

Not a sermon....just a thought