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Jeep Wj regear question

611 views 9 replies 8 participants last post by  cDee63 
#1 ·
Hi,
I just put a set of 33's on my Jeep and now it gets terrible gas mileage. I was looking to regear and when I looked at the regear chart it said for best gas mileage to put 3.91's in it. Right now I have 3.73's in it. I was wondering if regearing to 3.91's will be a noticeable and substantial difference or if I should go up to 4.10's or higher.
Thank you!
 
#3 ·
How are you getting the mileage number? You can NOT use the overhead readout. It's calibrated to the stock tires and will show far lower mileage because you're actually traveling farther than the computer thinks you are. More details here: 10.5 11.5 mpg on my 4.0 WJ.... what gives?

Also note that larger tires have a bigger contact patch, higher rolling resistance, higher wind resistance, and the lift has moved the axles down into the air stream to create even more drag. All of those will reduce fuel mileage a bit and no gears will fix any of them. There ain't no such thing as a free lunch. ;)
 
#6 ·
How are you getting the mileage number? You can NOT use the overhead readout. It's calibrated to the stock tires and will show far lower mileage because you're actually traveling farther than the computer thinks you are. More details here: 10.5 11.5 mpg on my 4.0 WJ.... what gives?
Nice discussion, but one little detail: circumference of a circle is pi*diameter.

Determining the change in odometer accuracy is just a matter of figuring the tire ratio. Ratio of 33/30 is 1.1, so if your odometer was calibrated for 30" and reads 100 miles, if you install 33" then you really went 110 miles. This means your fuel efficiency is really 10% higher than you thought. Not really a significant difference in my book, but still something to keep in mind.

I know my calculated mileage dropped significantly from 16 mpg to 13 mpg when I went from 29.5" to 30.5" ... a 3% change in circumference. That is more than just a change in odometer accuracy. Could not tell you why the big difference but like you say, there are multiple factors involved.

Possibly one of the big ones is the bigger tires reduce the available torque (by the same ratio) where the rubber meets the road, so you cannot pull as much or accelerate as quickly. I have not figured it, but I suspect this also translates to power. Bottom line is if you want to travel the same speed with bigger tires then you need to give it more fuel, reducing fuel efficiency.

Does regearing recover fuel efficiency? Idk, if it does then that is worth considering. I know after 220k miles I have been thinking about diff rebuild and that would be a good time for regearing ... If it is worth a couple mpg then it is worth more thought.
 
#4 ·
I wouldn’t make the effort for a change that small, I used 4.56 in mine with smaller tires than you and was quite happy with the results.

But your fuel mileage is not coming back, no matter what you do with gears. That’s a fantasy.
 
#7 ·
Assuming you got the 4.slow? You need to use an actual gear calculator and not the silly charts that don't factor in the engine, tranny and tcase.

I'd use this.

www.grimmjeeper.com

You lifted it, so nothing is getting your MPG back other than returning it to stock. With 33's on the 4.0 slow you should be looking at 4.56 or 4.88s. 4.88s if you are going to wheel and want to get your low end torque back. 4.56s would be good all around. If you got a trudy5 rear I'd be looking to swap out that useless boat anchor.
 
#10 ·
Cold weather will also kill mileage. In summer with stock gearing and 32's I get around 13-14 mpg calculating from fill ups, not the overhead readout, it is a bit optimistic showing 14-15 or so. In winter with 30" winter tires it drops, at last check I was getting a whopping 11.7 mpg.
 
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