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-   -   3.45 Axle Ratio (http://www.jeepgarage.org/f109/3-45-axle-ratio-30337.html)

BobT 10-30-2011 12:39 AM

3.45 Axle Ratio
 
Down-under they are offering an Off-Road Adventure II Package which among other things, includes a 3.45 axle ratio as against the standard axle ratio of 3.09. Does anyone know how the shorter axle ratio will affect fuel consumption? I expect it will make it worse but I can't find any figures on it.

If Jeep supplies fuel consumption figures for for their vehicles in standard for, they should also supply them for drive train options.

BobT 10-31-2011 05:00 PM

Re: 3.45 Axle Ratio
 
OK, no responses, but I think I have the answer anyway from http://www.trucktestdigest.com/axle%20ratios.htm . There is probably no noticeable difference in fuel economy. Nevertheless, it would have been good if Jeep confirmed this in their documentation. The change in axle ratio may be to adjust for the additional weight of the skip plates or to improve off-road performance or both.

AIR_RAM 11-21-2011 07:46 PM

Re: 3.45 Axle Ratio
 
I first apologize for this long post… I hope a few find it informing, although I’m sure it will raise some doubt with those who have a hard time doing the simple math or just dont understand overdrive transmissions and how low RPMs effect fuel economey.

The current gear ratios they offer are ALL mathematically lethargic and do just the opposite as there intended and fuel economy is not as good as it could be had they stepped up the ratios.

Keep in mind our JEEPS are NOT the first vehicles to get the MDS HEMI & 545RFE combo... There are MANY Chrysler vehicles to got it before us...

My experience with Chrysler and their UNDER Gearing and OVER Driving combinations are backed up with simple math.

RPMs alone do NOT dictate fuel economy. Lower RPMs do NOT mean better fuel economy as many think. Too low of RPMs causes the engine to LUG which in turn causes the PCM to ADD FUEL! To low of an RPM also causes the transmission to hunt for gears on small inclines or under light load.

It’s the load on the engine at ANY given RPM that has EVERYTHING to do with fuel economy. Chose a gear that will get you deeper into your torque curve and you will see improvements on fuel economy & overall performance.
Case in point, the two options you mentioned…

When you take into consideration our transmission overdrive ratios are .75/.67 and the fact that our tires are over 30” tall from the factory the math just gets interesting.
____________

Here is the SIMPLE math ANYONE can use to determine your OVERALL GEAR RATIO.

1) REAR GEAR x TRANS GEAR = OVERALL GEAR RAIO (OGR)

2) SPEED x OGR x 336 / TIRE HIGHT = RPM
____________

3.09 Ratio

REAR GEAR x TRANS OVER DRIVE = OVERALL GEAR RATIO
3.09 x .75 =2.31
3.09 x .67 = 2.07 There is no way on earth your MDS (4-cyl) will EVER hold that gear anywhere but going downhill…

SPEED x OGR x 336 / TIRE HIGHT = RPM
60MPH x 2.31 x 336 / 30” = 1552
60MPH x 2.07 x 336 / 30” = 1391 There it is… Not going to happen!
So your overall gear ratios in OVERDRIVE effectively become 2.31/2.07
____________

3.45 Ratio

REAR GEAR x TRANS OVER DRIVE = OVERALL GEAR RATIO
3.45 x .75 = 2.58
3.45 x .67 = 2.31 Your MDS struggles to hold this on anything but a flat road with the wind at your back!
SPEED x OGR x 336 / TIRE HIGHT = RPM
60MPH x 2.58 x 336 / 30” = 1552
60MPH x 2.31 x 336 / 30” = 1552

____________

Here is what happens when we use 4.56’s…
4.56 Ratio

REAR GEAR x TRANS OVER DRIVE = OVERALL GEAR RATIO
4.56 x .75 = 3.42
4.56 x .67 = 3.05

SPEED x OGR x 336 / TIRE HIGHT = RPM
60MPH x 3.42 x 336 / 30” = 2298
60MPH x 3.05 x 336 / 30” = 2049


To better understand what the OVERALL GEAR RATIO means… look at it as your actual gear ratio with a non-overdrive trans with a 1:1 ratio. Back in the day the 3.55 gears where considered a gutless Granny Gear with a non-overdrive transmission.

So with your 3.09 option your OGR in OVERDRIVE effectively become 2.31/2.07

With your 3.45 option your OGR in OVERDRIVE effectively become 2.58/2.31

Unfortunately it appears that Chrysler used the same math that landed them in bankruptcy that they used to choosing our gear ratios in our trucks... The math just does not add up.

When you add 30"+ tall tires to ANYTHING you need to add some mechanical advantage to the engine in order to turn them. But Chrysler did just the opposite. They added the tallest factory tires offered on any production vehicle and then added the highest overdrive ratios ever seen on a transmission... they tend to LUG the engine down adding up to bad performance & bad fuel economy.


From my experience gained from over 10 years working with the 3.7L, 4.7L & 5.7L with the 45RFE & 545RFE transmission combination we have learned that for best overall performance with these engines you want to aim for 2000RPM@60MPH. NOT 1999RPM@60MPH... You’re better off with 2100RPMs @60PH then 1999RPM@60MPH...

If your tire height is over 30" tall then 4.56's would mathematically be the best rear gear for OVERALL PERFORMANCE to include increased fuel economy.
If your tire height is over 34” Tall then you should be looking at 4.88 gears for the best rear gear for OVERALL PERFORMANCE to include increased fuel economy.
On my 2002 4.7L Ram with the 45RFE (.75 Overdrive) I went from 3.55 to 4.56 and I gained 300RPM@60MPH and 3-4MPG overall.

You will see a gain going from stock to a 4.10... BUT you could see greater gains by going to a 4.56....Selecting the best rear gear all depend on your tire height, Overdrive Ratio which give us the OVERALL GEAR RATIO.

If anyone has any questions please feel free to contact me.

SPEED SAFE, AIR RAM

MrWow 12-05-2011 01:04 PM

Re: 3.45 Axle Ratio
 
PM sent

ooja3k 12-05-2011 10:57 PM

Re: 3.45 Axle Ratio
 
So where can we get these gears ;)

jraptor04 12-08-2011 11:41 AM

Re: 3.45 Axle Ratio
 
My WK2 came with 3.47 and I cant imagine having anything lower. It would be a real dog. I have the same fuel milage as everyone else, about 12-14 city 20-22 hwy

AIR_RAM 11-11-2012 11:02 PM

Re: 3.45 Axle Ratio
 
Does anyone make rear gears for the 2011 JGC yet?

SPEED SAFE, NICK

ColdCase 11-12-2012 11:10 AM

Re: 3.45 Axle Ratio
 
I have not heard of any. Once in awhile the STR8 guys try to get enough folks together to fund the tooling costs, but so far just not enough demand.

Those serious about it and have OEM solid axles have been swapping the metric gear axles out for ford rear ends.

suzieque 11-12-2012 01:49 PM

Re: 3.45 Axle Ratio
 
Quote:

Originally Posted by AIR_RAM (Post 519434)
From my experience gained from over 10 years working with the 3.7L, 4.7L & 5.7L with the 45RFE & 545RFE transmission combination we have learned that for best overall performance with these engines you want to aim for 2000RPM@60MPH. NOT 1999RPM@60MPH... Youre better off with 2100RPMs @60PH then 1999RPM@60MPH...

how about the 3.0L diesel? is it good to run it at 2000RPM@MPG? or some lower RPM?

kkreit01 11-13-2012 03:34 PM

Re: 3.45 Axle Ratio
 
In the US, all Pentastars have 3:06 (way too high), all 5.7 have 3:45, 6.4 have 3:70. With the tow package, they all should have 3:92s -- like previous model Aspens/Durangos. Unfortunately, for us, we have no other option. We just have to keep mashing the pedal at every stop light -- which in turn uses more gas to defeat the purpose of the highway gears to begin with. My new Ram has 3:92s -- which are perfect.

ColdCase 11-13-2012 04:36 PM

Re: 3.45 Axle Ratio
 
Rear end ratio is only a part of the story. I had 3:73s in my Dakota and RAM and they were brutal on the highway. At highway speeds the engine felt like it was going to fly apart and mpg went into the toilet. I oversize the tires 10% on the RAM to make it reasonable which also improved gas mileage about 20% under every driving condition (many owners added overdrives). I just lived with it on the Dakota.

In the RAM's case, the manual transmission gearing had a lot to do with it. We never used 1st gear, always starting out in 2nd unless pulling heavy. So when you talk about ideal axle ratios, you cannot ignore transmission gearing.

The WK2 has very tall overdrive gears (both top gears are overdrives). The gear ratios are so wide its leading to many of the gripes here, nothing that wrong with the axle ratios. Downshift the transmission, only use 4th or 5th, it will feel like you have steeper axles. I'm not sure what the current auto trans ratios are in the RAM, but I hear the spacing is a lot better overall than a few years ago.

Personally I like the gearing on my V8, I think its pretty close to ideal. I don't really like any auto transmission, but the WK2 is annoyingly slow to respond at full throttle sometimes. I don't think axle ratios are not going to change that.

bit50 11-13-2012 04:46 PM

Re: 3.45 Axle Ratio
 
It would just be nice if there were options!!!


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