If you upgrade gears, will it improve gas mileage? (if so, by how much and what gears would be more then suitable with the 33's, thinking about 35's in the future?) I have a 2dr, 4:10's with 33's

 

Most likely the best gas mileage you will get would be from a stock JK. Of course if you go to bigger tires, and keep the same gearing your MPG will go down, as will your performance. With that in mind it would be advantagious to try to get back to the stock numbers. Larger tires would mean regearing to get back to as close to stock as posible whatever that gear ratio would be is what is needed. On the other hand, I doubt once larger tires are installed you will ever get back to the MPG you were getting with a stock set up. You have added more weight and friction to the JK, and the power from the engine has remained constant.

 

33's and 4.10's are fine, if you are going to go to 35's get 4.88

 

The 4.10's will work with 33's. You didn't mention if auto or stick. Re-gearing is pretty expensive and you don't want to have to do it twice. Some people have made the mistake of regearing from 3.73 to 4.10 and then realizing that they needed 4.88 or even 5.13 as they changed their setup.

Drive the 33's with the 4.10s. Bigger tires may get you over a few more obstacles but good technique and proper gearing will get you wherever you want to go.

When you move to 35's keep the 4.10s for a month. You gut will tell you what you need to do.

 

Well, sort of, if you mean by "upgrade" to get 3.21's.



You see, when the factory engineers were trying to make a somewhat reasonable (For a jeep...) CAFE MPG number...they tested out rpm ranges that produced the best fuel economy, and came up with a 3.21 ratio with 32" tires.

If you have 33's and 4.10 gears, you are already cruising at a higher rpm than ideal for best gas mileage.

Swapping on 35's will lower your rpm about 5.7% from the 33's figures.

That will STILL yield a higher rpm than the factory 3.21 gearing and 32's.

So - getting even DEEPER diff gears will NOT improve the mpg.

Getting 35's instead of 33's, now, might.

35's are a break point tire size for the JK...its positioned so as to act as a deeper overdrive, but not so big that rolling resistance has added up too badly....but if you went to 37's, BAM! Gas gets guzzled WAY faster, the effective final gearing is high enough to need to drop a gear or two to maintain speed on grades more than normal, etc.

That said...um, I did not buy the JK because It looked like a Prius...I'm only a little concerned that gas is going to be $4-ish/gallon, etc...Because I knew that was a possibility for every jeep I've ever had.

IE: Gears the produce good fuel economy are NOT the most pleasant to drive with, unless you are very patient, etc.

So - its a trade off, power vs economy...what's most important to you?

For the best fuel economy, you would be cruising along at ~ 1,780 rpm at 60 mph...3.73's would up you to about 1,970 rpm, and 4.10's would put you around 2,150 RPM at 60 mph. (Remember, with the speedometer being WRONG with the 33's...going 60 mph in reality, and 60 on the speedo, will not match anymore...)

Continuing to deeper gears will of course continue to RAISE your RPM for a given speed, and, FURTHER hurt your mpg....but, it WILL HELP your performance.

As recommended by others above, and by myself ad nauseam...drive on the 4.10's, and go with your gut as to your priorities when the time comes to pay at the pump, and/or with your patience being a virtue thing.

 

TEEJ,

I like reading your posts, lots of good info!

 

I've heard the argument on power vs. economy and agree with the reasoning behind it but it seems that in real life situations that a gear change to 4:88 or 5:13 yeilds better mpg and performance. At least that is what people are saying.

 

I changed to 4.88s and my MPG dropped by 2.

 

I've been running my 4.10's with 35's since the day my Rubi left the dealer lot. It's fine, not great for mountainsous regions, but it's very good around town and on the Highway. I even use 6th gear in town. It's not that bad at all or I would have changed mine already. I do plan to go with 5.13's eventually so when I up to 37's, I 'll already be good to go.