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-   -   advice on gearing (http://www.wranglerforum.com/f282/advice-on-gearing-239459.html)

turnbull 05-01-2013 10:52 AM

advice on gearing
 
So first let me start off with what I know for certain: I have an '04 Wrangler with the 4.0L, 5-speed manual, D44 rear, and 35" tires. And here is what I think I know: I am running this on a 3.07 gear ratio. I don't have my Jeep right now and I won't have access to it for about 2 more weeks, so I can't check the rear diff. However, judging by the RPMs vs. MPH I got, I figured it must have 3.07 gear ratio. By the way, I bought it like this, so no hatin :)

At around 2150 RPMs, the speedometer reads 60 MPH, but since the speedometer is synced to stock tires, I am actually driving around 74 MPH. Honestly though, I can maintain speed pretty well in 5th gear, even on slight grades.

So I was thinking about regearing, since I have seen suggestions for 4.88 gears for 35s which is significantly higher than what I have. But one thing to note is that I will be getting 30" tires for highway driving, and most of my driving will be done on those tires, so 4.88 gears wouldn't really make sense. I was taking a look at this chart to get an idea of what gear ratios to go with: Tire to RPM Ratio Chart | jeepfan.com

I was thinking about 3.73 gears, since I figured that would be a good balance between 30" and 35" (with more favor towards 30" since they will be used more), and I know that was a stock option for Jeeps anyway. So my question is, how will increasing my gear ratio to 3.73 affect my mileage for both sets of tires (30" and 35")? Will it increase or decrease? Right now I am getting ~15-16 MPGs with the 35s (I don't have 30s yet).

According to the chart I posted, the yellow-highlighted RPMs indicate "better gas mileage," but does this mean optimal gas mileage or just better than "factory ratio"? Because if it were optimal gas mileage, I don't know why 3.07 gears would even be an option since it is not even close to the optimal mileage.

Gunner 05-01-2013 11:04 AM

My guess is your going to want to be some where in the 2250 to 2400 rpm cruise range on your 30 inch tires. Pick the speed you drive and gear accordingly.

SinfulDragon 05-01-2013 11:13 AM

I thought the D44 came with 3.73 or better gears...

turnbull 05-01-2013 11:36 AM

Quote:

Originally Posted by SinfulDragon (Post 3705908)
I thought the D44 came with 3.73 or better gears...

I thought the same. That's one reason why I am confused. I wish I could just check, but I can't so I just did the math and came up with ~3.07.

Quote:

Originally Posted by Gunner (Post 3705908)
My guess is your going to want to be some where in the 2250 to 2400 rpm cruise range on your 30 inch tires. Pick the speed you drive and gear accordingly.

So that is the optimal gas mileage range?

PiePuncher 05-01-2013 12:22 PM

I would get 4.10/4.11. It will increase your acceleration and you will still have a good top speed. It will also allow you to increase your tire size quite a bit if wanted. There will b a big difference in 3.07 and 4.10/4.11.... Is there not a tag on your differential that states what your gearing is??

530ktm 05-01-2013 12:30 PM

You can not possibly know what your gas mileage is with out having the speedometer calibrated accurately and you already said it is a mile off. Also, going from 35 to 30 inch tires is a huge difference. If you do any kind of off road driving you will not be very happy with anything but 4.88 gearing running 35 inch tires. And forget driving on 30 inch tires with that gearing. Not sure what you are trying to accomplish here but have you thought about going somewhere in the middle and run 33 inch tires and gear properly for that set up, like 4.56 gearing. This will run great on road and off road. I do not see the benefit of changing from 35's to 30's for any reason.

PiePuncher 05-01-2013 12:43 PM

3.07 turning 35's.... U sure u have 3.07. Your jeep would be a slug with 35's. A smaller tire, less lift, would increase gas mileage... Also, if u r running 35's, won't little tires with lift look funny? Reading 530ktm post got me thinking and there is lots wrong w the whole picture..

turnbull 05-01-2013 12:49 PM

Quote:

Originally Posted by 530ktm (Post 3706257)
You can not possibly know what your gas mileage is with out having the speedometer calibrated accurately and you already said it is a mile off. Also, going from 35 to 30 inch tires is a huge difference. If you do any kind of off road driving you will not be very happy with anything but 4.88 gearing running 35 inch tires. And forget driving on 30 inch tires with that gearing. Not sure what you are trying to accomplish here but have you thought about going somewhere in the middle and run 33 inch tires and gear properly for that set up, like 4.56 gearing. This will run great on road and off road. I do not see the benefit of changing from 35's to 30's for any reason.

Well I calculated the MPG based on the length of the trip using MapQuest. And there appears to be a bit of confusion: I am going to have 2 sets of tires. 1 set for highway driving and 1 set for off-road. I can't have just one set of 33s because MPG is still going to hurt. Although what I might do is when it is time to change my 35s, I might get 33s instead, but I don't know how that will look with a 6" lift.

I don't see why I would need such a high gear ratio for off-roading. It's not like i'm gunna be driving in 5th gear off-road. Plus I already have plenty of power in lower gears.

Quote:

Originally Posted by PiePuncher (Post 3706257)
I would get 4.10/4.11. It will increase your acceleration and you will still have a good top speed. It will also allow you to increase your tire size quite a bit if wanted. There will b a big difference in 3.07 and 4.10/4.11.... Is there not a tag on your differential that states what your gearing is??

Yea but I don't plan on increasing my tires any more; 35s are plenty big enough for me. And while I know there will be an increase in power, I am worried it might hurt my MPG (mostly on the 30" tires). Like I said, there might be a tag on the diff, but I won't see my Jeep for another two weeks so I can't check it

turnbull 05-01-2013 12:51 PM

Quote:

Originally Posted by PiePuncher (Post 3706304)
3.07 turning 35's.... U sure u have 3.07. Your jeep would be a slug with 35's. A smaller tire, less lift, would increase gas mileage... Also, if u r running 35's, won't little tires with lift look funny? Reading 530ktm post got me thinking and there is lots wrong w the whole picture..

No I am not sure. However, I am going by this equation:

RPM * Tire Diameter/(MPH * 336)

So the RPM is ~2150, MPH is ~74, and Tire Diameter is 35. You can check the numbers yourself but I'm pretty sure I did it correctly. Unless this is the wrong equation of course.

And yea, 30" tires will look funny, but I am still keeping the 35s so I need the lift still.

Jerry Bransford 05-01-2013 12:53 PM

I believe you would be disappointed with performance no matter what ratio you choose to try to work with both 30" and 35" tires. Those tire sizes are just too far apart for any ratio to be even a good compromise. I'd pick one tire size to live with & gear for that. Or at least two tire sizes that are closer together.

turnbull 05-01-2013 12:59 PM

Quote:

Originally Posted by Jerry Bransford (Post 3706342)
I believe you would be disappointed with performance no matter what ratio you choose to try to work with both 30" and 35" tires. Those tire sizes are just too far apart for any ratio to be even a good compromise. I'd stick with one tire size & gear for that. Or at least two tire sizes that are closer together.

I'm not looking for an ideal between the 2 sizes. Basically what I am looking for is to increase my gears as much as possible w/o decreasing my MPG on the 30" tires. I am actually fine with the gears I have right now for the 35s. Sure I have to shift to 4th if the grade is steep enough, but its no big deal for me. Obviously increasing the gear ratio would be better for the 35s, but I'm not going to do it if it hurts my MPG on my 30s.

kyjeep 05-01-2013 01:01 PM

Quote:

Originally Posted by jerry bransford (Post 3706342)
i believe you would be disappointed with performance no matter what ratio you choose to try to work with both 30" and 35" tires. Those tire sizes are just too far apart for any ratio to be even a good compromise. I'd stick with one tire size & gear for that. Or at least two tire sizes that are closer together.

+1

SuicideSaints 05-01-2013 01:03 PM

I am not sure how you came up with that gear ratio? How did you do it, with the math equation? If you are using the (rpm x tire diameter) / (mph x 336) you have to make sure you are reading your true speedo on the gauge. Unless you have recalibrated the gauge or using gps speed that equation is going to be off and you will incorrectly calculate the gear ratio. I would say there is no way you have that gear ratio. I would say you have 3.73 gears. I would agree with everyone else saying that get a 33 in tire and not two, the difference between 30-35 is huge. I only lost about 1-2 mpg when I regeared and went to a 35 in tire. I was pulling in about 17 now with 6+ in lift and 35 in KM2s I am averaging 15. I regeared to 4.56 and love it, 4.88 would be a good choice for 35, if you go with 33s 4.56 is a good choice.

SuicideSaints 05-01-2013 01:06 PM

Ok, I see the equation you used, it is the correct equation. But, your speedometer is off on your jeep and that is not the true speed your jeep is traveling at that rpm. So that is throwing your calculation off. You will need to know true speed using GPS.

SuicideSaints 05-01-2013 01:08 PM

Sorry one more thing. This equation only works if you are in 4th gear when you take the rpm reading. In 4th gear the ratio is 1:1

turnbull 05-01-2013 01:27 PM

Quote:

Originally Posted by SuicideSaints (Post 3706400)
Sorry one more thing. This equation only works if you are in 4th gear when you take the rpm reading. In 4th gear the ratio is 1:1

OHHH. That explains everything. I have been using the RPM value from 5th gear, but I was using the correct speed (74 MPH reads as 60 on my speedo). I don't know the RPMs at 4th gear. But my guess is that the my gears are 3.73, possibly higher. That is fine by me, and so I do not plan on regearing anymore.

SuicideSaints 05-01-2013 01:38 PM

Yeah, that 4th gear is key. I messed up the calculation when I first was trying to figure my rig out. I am pretty sure you are sitting on the stock 3.73 gears.


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