Yet another gearing question.

JL Rockies · **Posted:** Sun Jan 27, 2008 2:27 pm

If/when I re gear, will my RPM go up or down on the highway?

I have 32" tires on my CRD now and still have the stock gears. Outside of having to turn off the OD on steep grades to keep the tranny from downshifting, everything is cool. I drive for many hours with the cruise set for 80 mph (verified by GPS) and RPM's are about 2200 with OD engaged. It would seem that 4.10 gears are the only thing that can be changed relatively easily on the KJ.

Would the RPM's be affected at all with these gears? I already had a 5 mpg drop when I lifted and added tires and I am not too keen on doing something that would make it drop even further if you know what I mean.

I am clueless with this stuff and some calculator I found on line makes me think that this could be bad for mpg. Clue me in, my black belt is in the 12V arts.

Thank you.

Diggerfreek · **Posted:** Sun Jan 27, 2008 4:01 pm

depends on your current ratio....if you have 3.73 and went to a 4.10, you will increase torque to the wheel, but your motor will turn at a faster rpm to maintain the same speed...now if you went to a 3.21, torque is less at the wheel but the engine will turn at a slower rpm to maintain the same speed...in other words, if you gear to a 4.10, you may have to turn the motor at 2600 rpm to do 80 where at with 3.21, it may turn at 1500 to run 80....hope this helps you

tommudd · **Posted:** Sun Jan 27, 2008 4:53 pm

Since you went to a larger tire you need to re gear to get the engine/ drivetrain back into their sweet spot. Every vehicle is set up to perform better at a certain RPM/speed etc so with you installing larger tires you need a lower gear ratio (and since we are stuck with 4.10s being the lowest you should go that route) Many times with going to the lower gears after installing larger diameter tires you gain back the mileage you LOST . Talked to a guy the other day running 37s on his LJ, after intalling the lift and tires went from 15/16 mpg to 9/10, swapped in 5.13s and now back to 15/16 so it can help a lot!

ATXKJ · **Posted:** Sun Jan 27, 2008 6:24 pm

I wouldn't change to 4.10 unless you're towing something - that would increase your engine rpm to 2400 rpm at 80.

however if you want good mileage you need to slow down - the sweet spot for the diesel is 1800-2000 rpm -
yes it'll run 2200-2300 rpm all day long but it uses a lot of fuel to do it -
Liberty aerodynamics aren't helping anything at 80.

On a diesel - if you want better highway mileage you might look at finding some of the 3.54's off of a 6 speed.
that would drop to a 2100rpm cruise - as long as you don't have to kick out of overdrive that would help.

however at the same time 3.54's will kill acceleration in town, and stop & go mileage.

JL Rockies · **Posted:** Sun Jan 27, 2008 7:00 pm

I typically set the cruise to 5 mph over the posted speed. Much of the highways through Northern UT, Southern ID, MT and WY are posted 75 mph. There are miles and miles between the miles and miles I have to travel. I don't seek to be that fastest moving object on the road, but it puts me in the middle. I could drive 65 mph in the right lane with the semi's, but that would also add a lot of time to my ETA's.

So if the RPM's would be greater with 4.10 gears (which is what the calculator told me) it would make sense that mpg would suffer.
I have also heard from many what the honorable tommud has stated: I would get some of the mpg back by going with the 4.10 gears. :?:

What about off road performance? Would the greater torque of the 4.10 gears be an advantage? There have been a couple of time I have had to do a brake stand to get over some obstacles. I let the turbo spool up and let off the brake. I could not get over some of the waterfalls on Golden Spike without doing this.

Thanks for the advice so far dudes.

AZScout · **Posted:** Sun Jan 27, 2008 7:19 pm

4:10 gears with 32" tires will bring you back to stock.

Jeepjeepster · **Posted:** Sun Jan 27, 2008 8:40 pm

What were your rpms at 80mph before the tires? That will tell you if you need to re-gear or not.

Gosh, I run 2500rpms at 80mph. Wish I had a crd. :evil:

jason thompson · **Posted:** Sun Jan 27, 2008 9:06 pm

you need to regear
you could have a 400 HP V8
if you have 40" tall tires and 3.07 gears it will be a dog
go to 5.13's and it will be a champ out on the trails

My brothers YJ is a great example of why regearing works so good
I-6 and 5 speed with the stock 29" tall tires and 3.07 gears he got ~15 mpg
lifted and 33's he could not use 5th even at 70 mph ,which is FAST in a YJ
also his MPG dropped to under 10 mpg
he regeared to 4.56 and is now back to ~14 mpg and can burn 33 X 12.5 swampers on pavement
I plan to go with 4.56 when I put 33's on my TJ

I also plan to put 4.10's on the KJ
along with a Detroit locker rear and TT front

Uffe · **Posted:** Mon Jan 28, 2008 4:22 am

I'm running 4.1 in my KJ CRD. I get proper MPGs at revs of 1800. I can choose between speeds of 57ish mph or 50ish mph (2nd and 1st OD) for best fuel economy. You need a little trick to lure it to run in 2nd OD at 1800rpm with 4.1 gears, first go up to 60mph to get into 2nd OD then back off to 57ish mph. My EVIC shows me about 33MPG at those speeds with no wind and level driving.

In reality a trip on country roads with a few roundabouts and a little highway driving turned out 24.75MPG at best recently. Mostly I get about 21MPG on a full tank, with harsh weather and plenty of cold starts.

JL Rockies · **Posted:** Mon Jan 28, 2008 1:35 pm

I was getting 21 MPG before lift and tires @ 80 MPG with my Edge module in fuel saving mode.

ATXKJ · **Posted:** Mon Jan 28, 2008 2:27 pm

The lower 4.10 gears will only help if you don't have to torque to stay in overdrive and have to downshift.
however if you're staying in od - the 4.10 will make it worse because you have to rev faster.
The 4.10 will be better offroad and in town acceleration - but not highway.

aerodynamically - you're a lot worse with the lift and tires - and the stock Liberty wasn't great to begin with.
some of these ideas might help
http://www.recumbents.com/car_aerodynamics/

http://www.dodgeforum.com/m_447500/tm.htm

ATXKJ · **Posted:** Mon Jan 28, 2008 2:40 pm

JL Rockies
on your mileage calculations are you correcting for the tire size?
That's a 10% change stock to 235/85 - and the odometer doesn't recognize that - it only understands revolutions.

chefdab · **Posted:** Mon Jan 28, 2008 3:08 pm

JL Rockies wrote:

I was getting 21 MPG before lift and tires @ 80 MPG with my Edge module in fuel saving mode.

I assume you mean at 80 mph. Your fuel economy will really start going downhill after 60-65 mph though. Slow down a little and watch your economy go up about 4-5 mpg. 4.10s will give you back a little power so you don't have to downshift as much. Yes the re-gear will help but not as much as lightening up on the skinny pedal.

JL Rockies · **Posted:** Mon Jan 28, 2008 3:51 pm

I calculate MPG by dividing miles traveled (according to GPS) by the amount of fuel I purchased. I do this every time I buy fuel. Last week I traveled up to western MT, across the state to eastern MT , down to Casper, WY and then across WY back to UT.

Lowest MPG recorded was 16 mph and the highest was 19.4 mph on this trip. The higher mileage was probably due to the lower speed on I 80 West across WY. On Sat there were severe gusting winds (a WY specialty) blowing snow across the highway. The semis were traveling in both lanes at about 25 mph due to snow drifts and their high center of gravity.

Sometimes I hit such strong headwinds driving along I 80 I have to turn the OD off just to keep the tranny from downshifting constantly. It's crazy out there sometimes, I don't know how the truckers do it.

ATXKJ · **Posted:** Mon Jan 28, 2008 4:58 pm

The GPS takes the odometer calculation out of the mix - however don't forget to put winter fuel in as a variable

back to your original question - if you have to turn the OD off - then the 4.1 would probably help

the tranny ratios
3rd 1:1 - effective final ratio 3.73
4th 0.75:1 effective final 2.79
5th 0.67:1 effective final 2.49

so DO off = 2.49 to 3.73 ~ 30% drop
with 4.10 effective ratios go to
4.10
3.00
2.75

so 2nd OD with 4.10 = 1st OD with 3.73

but gears won't fix the aerodynamics so that'll still be an issue

Yet another gearing question.

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