Hey guys,
As it says above, I just upgraded to 4:88s. i am in the break in period right now, so i have been unable to really push it yet. It seems I am reving really high in the first four gears just to get up to speed. My jeep is a 2 door x manual with 35'' tires, and a 2.5'' Lift. My question, is my gas milage gonna be crap now that i am revving higher? This is my daily driver, and do mostly city driving, with a couple highway trips here and there. I regeared to get better mileage with the increase in tire, but am a little concerned when i am seeing the rpms on the highway. I am new to this, so any advice from experience would help. would 4:56's been better, or did i make a good decision with the 4:88s?
thanks for the help guys,

 

Of course you are going to rev higher with 4.88's compared to 3.21's.. MUCH higher. I think you are just in the process of getting use to it. Give it a little time and you will grow to love your new gears.

 

u have a 6 speed, that means u control the shift points, shift sooner and start out in 2nd if 1st is to low for you. I ihave 4.88s and 34 inch tires and almost wish I went with 5.13s. Gearing is your friend off road. If your not taking ur jeep off road and wanted better mileage, u prob should have went with 4.10s. sorry to say 4.88s will not do anthing to your mileage except make it worse. My mileage went from 14 mpg on highway to 11 on highway when I regeared. gearing low (numericaly higher) is about performance, not gas mileage. U will learn to love your gears performance wise thoug, I regeared in February this year, and I was nice to be able to pass other vehcile on 2 lane roads other than tractors lol. off road is awsome, crawl ratio is GREATLY improved.

 

You're just shifting too soon.

I assume you shift, what, around 2500 RPMs or so? Now you're getting there at lower speeds and even with the regearing, you still have the same amount of power at the wheels at 2500 RPMs as you did before. It must be an odd driving experience.

The power output of the 3.8 increases dramatically as RPMs rise. The goal of regearing to replace the stock "feeling" is to put RPMs where they were on stock tires. Almost nobody does that though, including you (4.10s would've been closer). The real goal of a regearing is to move more rapidly up through the RPMs than you did even in the stock set up so you can access more of the 3.8's power.

Start shifting at 3500 and see how you feel. Let it spin out to 4000 or even 4800 or so when you're trying to pass somebody or merge on the highway. It won't hurt your engine and you'll have much more power than you did on the 3.21s, when it would've taken much longer to rev up to those RPMs in low gears and they would've be unaccessible in high gears.

 

Thanks for all the info guys!

 

I'm sure a dyno chart for a 3.8 can be found somewhere on this site.

Check it out and notice how rapidly horsepower at the wheels rises as RPMs rise. In its stock form, the 3.21s had you turning very low RPMs for a given speed. The result was that you had very little power at your wheels at that speed. You made this worse by adding bigger tires, which lowered your RPMs even more.

By regearing to 4.88, you got back to where you were before the tires and then some. Whereas your prior RPMs at 70 mph on 35s were probably around 1800, they'll now be at 2700. Your engine is putting much, much more power on the ground at 2700 RPMs. I don't have a dyno chart handy, but I'll bet you've about DOUBLED your horsepower at that speed. That'll be true of other speeds as well, because whatever RPMs you used to be at for those speeds will now be much higher.

The only catch to all this is that you've got to get used to turning those higher RPMs. Previously, I'll be you hardly ever got over 2500 RPMs while driving. You maybe got up to 3k to shift. Well, that's different now. At 75 or 80 on the highway, you'll be spinnng 3k in 6th gear. I'd say add 1000 RPMs to whatever you used to consider normal.

Thankfully, the 3.8 does well under higher RPMs. It will however probably ding your gas mileage. Though FWIW, you'll lug the engine less so that will help gas mileage. You'll have to see how it balances out.

 

kandn.com/dynocharts/57-1553_dyno.pdf

 

There you go.

You're putting about ~40 horsepower on the ground with 2000 RPMs. At 3000, you're putting down ~100 horsepower.

That loosely correlates to your prior and new RPMs at 70ish mph. In other words, at 70 mph with your old gears in 6th you were only getting about 40 hp to the wheels. With your new gears, you'll get about 100. That's a big d*mn difference.

Of course, the "downside" is that your RPMs are going to be way higher. With your old gears, you could probably barely hear the engine as it casually spun 1800 RPMs in 6th at 70 mph. Now you can definitely hear it hum at 3000 RPMs. Another "downside" is that you're going to fly through gears in your transmission--1st gear for example is only going to last for a few seconds before you've got to upshift to 2nd. Some folks mitigate this issue by, as mentioned above, just starting in 2nd.

A small price to pay for the massive hp improvement in my view.

 

Thanks man for the info. took the jeep on a long drive on the highway and rural roads, and really have enjoyed the new gears. I think i needed to get used to it at first with the new way of driving aswell. Thanks for the info.

 

Some food for thought*: Regearing, for most of us anyway, is done to improve highway performance. After all, the JK is not a street racer, so shift points to optimize quarter-mile times are pretty ludicrous. And we have a two-speed transfer case to help with the off-road work. But, it's on the highway where performance lags, and this is probably most notable when cruising up hill and/or into high winds, and when passing another vehicle. So, ideally you will place that curve of the torque curve where it just begins that last bend to near level at your expected passing speed. That is, if you figure you'll hit 80 MPH when passing a vehicle, figure out your tach for a 70 MPH cruise speed, then back-calculate into the desired gear ratio using your tire size and tranny type. That will give you some kind of idea of the maximum you want that bend at so you don't run out of additional "oomph" when you really need it. Of course, you always have the option of shifting down, but then getting away from that as much as possible was the entire point of the exercise and expense of regearing, wasn't it?

*I have no particular expertise for what I just wrote. But, it makes a hella lot of sense to me.