Loadbreak

I have a 2-door Auto w/ 33's and 5:13's.(33's that actually measure 32.2). Its a little steep for me, but I plan on 35's so for right now its OK. But if I knew that I was going no taller than 33's... forever...then I would go to 4:88's. Tha chart listing is about right, 2600-2700 at 70..its a little buzzy, but it wheels great. Good luck in your decision.

grady

I would like to see this same test with a set of 35's. This is where I am now, trying to determine gearing and tire size for my auto unlimited before I am sorry after the fact. Great info.

troyboy

This useful to anyone? It shows both 3rd rpm and 4th rpm for all the gear ranges and tire sizes for 70mph




here is another one that shows different speeds.

theosdad

Thank You to everyone for the awesome responses. I Love this forum. I really do plan on staying with 33's. I believe 4:88's are the way to go. I only tow about 4 to 5 times a year anyway.

JPop

I certainly believe there are people out there that have enough experience driving both automatics and 6 speeds to have a very qualified opinion, despite their primary vehicle being a 6 speed. I for one, have put some decent miles on both as has planman.

Everything as laid out points to 5.13s and while 4.88s aren't going to be the end of the world, they are going to fall shy on the towing demands.

Much of this is just simple math and science. It doesn't approach art and the 5% difference between 4.88s and 5.13s are nominal. That said, and even with the 4-5 tow trips a year, that's what you build to, not a set up that leaves you short of fuel when you leave port and you encounter heavy seas.

All too often I think these posts are just looking for validation of what's been preemptively decided.

theosdad

You are right. I cant lie. I feel if I get the 4:88's I will wish I went with 5:13's. My biggest fear is a huge increase in rpm on the freeway where I spend almost all day. I dont want to get 10 mpg with normal driving, towing that would be fine.

JPop

Sorry about the somewhat terse response. Long day walking a trade show, dealing with the airport and a 4 hour flight home.

Formerly having a fairly long daily highway commute and putting a lot of miles on the JK every week I was able to do lots of rpm testing at various speeds and in different gears on a 6 speed. I did this with both 32s and 35s. I also listened to hundreds if not thousands of responses of how people felt their new gear sets responded in both automatics and manuals. The biggest variance was what type of transmissions people had, and an overall tendency for the gear sets to be recommended for an automatic would pull to the side of not steep enough and the manual transmission recommendations were often too steep.

That is why I built out the gear chart that is seen in the FAQ. It combines results from the various dynos that are available, lots of end user feed back and makes use of my many years of racing, albeit with a very different set of rules to achieve the best results. In drag racing the rule is pretty much over convert and under gear, while here and on this platform it is pretty simply to not be afraid to twist the motor a bit. There isn't anything more than a nominal efficiency hit to take (we're talking tenths of mpg) from 23-2700 rpm. Under 2300 rpm and then you start bogging things down with simple things like highway overpasses and minor grades, which for the automatic drivers means unlocking the converter more and potentially more kick downs robbing efficiency.

So look at the gear chart in the FAQ. Examine closely the very minor increases in rpms between 4.88s and 5.13s. Look at the Dyno charts, (hypertech has a good one) and see the power difference 125 rpm means (in round numbers 4-5 hp). Go into this knowing that biggest thief of efficiency and performance for automatic transmission owners is unlocking the converter and kick downs. That extra 5% of power can make a great deal of difference in the frequency of kickdowns, gets the converter locked quicker and as many have reported actually increase performance.

That's all I have for you and because the scenario as laid out included towing I would have recommended 5.38s had you been the owner of a D44 front axle. Don't fear the gear and best of luck however you roll.

theosdad

JPOP you are awesome. You have convinced me which takes alot. I finally realized that the rpm difference is so minimal its negligible. What really sold me was today I set my cruise at 65 and then again at 70 mph. What was wierd was that I got better gas mileage at 70 due to the decrease in the tranny downshifting and holding its gear. Thanks again for all the work you have done on the chart.

JPop

Glad I could be of help!

I wish I could explain things a little better, always a case of understanding something and unable to effectively communicate it to someone else, or at least not in a thousand or so words. One of the big things that being a little steeper in your gears does is at cruise speeds you are running a little more negative pressure in the manifold. Being up a little in the rpm range doesn't necessarily mean your burning fuel at the same rate as an rpm increase, but what it does do is place your torque on pretty much immediate demand.

With Torque being the hammer when you encounter a grade or a headwind, operating at moderately higher rpm will make more torque available when needed. If you were operating at a lower rpm, the throttle blade is already open wider, and when you encounter resistance there just isn't as much of reserve in torque and horsepower. The unfortunate downside of this, is when there isn't enough power or torque the way it get's put together is via unlocking the converter and the next step is actually a kick down.

In even more basic terms torque is the hammer and horsepower is the come along. Imagine a hill that you're going to encounter and the first thing that's really going to help you get up it is similar to getting whacked in the ass by a mallet. You might be able to make it up the hill with the come along but the hammer just makes the job easier and starts things moving. Add in a trailer and you need a bigger hammer.

Anyway, keep us posted on your results as I'm sure you'll be happy.

theosdad

Man you should write a book on this stuff. You make it easy to understand. Thanks again.