I might just have to try mine out tommorow and get a video. lol

I was trying to call my cousin tonight and ask him how big of the difference is between the two gears. I would think it should be pretty dang even at a 30mph start. I think from the dead stop the 3.92's would get the best of it. It is only a little software blocking that limitor. lol

I am not an export on gearing but I would assume the 3.21's will top out faster? You always lose topend on a lower gearing.

 

In my opinion, if you are doing mostly highway driving the 3.55 gears are perfect. I'm the same way. I drive 60 miles round trip every day, and 95% of that is on interstate. The 3.55 gears are definitely the better choice and are more than capable of doing anything you will ask the truck to do. At 75mph with the 3.55 gears my truck runs right around 1800rpm and at 80mph it's sitting right on 2000rpm. With 3.92 gears it will be higher than that. Higher rpm = worse gas mileage when it comes to highway traveling.

I think the 3.92 gears are better suited for towing or very mountainous areas.

On my very first tank of gas I got 15.1 mpg. That was actually calculated, not going by the computer. I'm on my second tank right now and so far I'm sitting right around the 15.4 mpg mark according to the computer which I reset when I filled up. This tank has had a mix of probably 70% highway to 30% city/two lane country road driving. I expect once it's broken in some more (I have 680 miles on it) it will creep up in the 16 mpg area and if I could manage to drive only 70mph it would increase a little more I'm willing to bet.

 

Some of you guys are over analyzing the fuel milage thing. Lower gearing means higher rpm, not lower mpg. RPM does not burn fuel, air burns fuel and that is controlled by your right foot. There is no appreciated difference in mpg on the highway.

As far as top end with the higher gearing. If there was no limiter, you would most likely have a higher top speed with the lower gearing. The reason is the hp/torque curve of the engine. Speed means wind resistance and a large truck has a ton of it. If the engine is not making enough torque to overcome the wind, your speed will be limited by the resistance of the wind. Classic case in point is that a full dress touring motorcycle will ususally be faster in 4th than 5th gear. In some cases with HD motorcycles, that is not the case because of their long stroke and torque. Look at a dyno chart of torque and HP curves. With internal combustion engines, highest speed will usually occur at the peak of torque.

 

Don't start blaming the right foot lol. Last night I was looking for the different times from 0-60mph and in the 1/4th mile with the different gears. Couldn't find any info though.
I couldn't even find any videos of the 09 Ram yet.

 

Since this thread is about mileage, one should be aware of when the fuel saving MDS kicks in and out. Does MDS become inactive at 70 mph? Keeping the speed down to where you are running on 4 cylinders seems most important for mileage. Does a 3.55 or 3.92 relay less stress on the Hemi so it can stay in 4 cylinder mode longer, perhaps on slight grades or against wind, say when cruising @ 60 mph on the open highway?

 

You are going to have to explain to me how you can have higher rpm, and more air, but not burn more fuel. They all go hand in hand. You don't just add more air and not more fuel I'm not being a smarta$$...seriously needing an explanation here...lol. Don't get me wrong, I understand some of your logic, but if you are going the same speed, but turning a lower rpm, how can you NOT get better gas mileage?

My point is this, I'd be willing to bet you could take two identical 09 Rams. One with 3.55s and another with 3.92s. Set them both at 75mph and look at the rpm the engine is turning. The truck with 3.55s will be turning a little less rpm. How much, I have no idea. With regards to your point about drag, it would take a pretty strong and steady headwind to affect rpm/speed. It's a variable that just wouldn't come into play that often. Tell me how in this instance, a truck with the 3.55 gears wouldn't get a tad better (I will admit it would be very minimal) gas mileage?

 

You are asking good questions. Higher RPM does not mean more air. The air volumn is controlled by throttle position. The more HP it takes to move your vehicle through the air or up the hill, the more throttle you have to use. A truck with 3.21 gears with have to use more throttle to pull a load up a grade than one with lower gearing. The higher gearing will be operating at lower RPM, but burning more air and gas by the throttle position. If you have less torque, you must make more HP and HP takes fuel while torque does not. Diesels get better mileage because there are more BTUs in a gallon of diesel, but also because they make much more torque than gasoline engines due.

At cruise speed on level ground, the 3.21 will get very slightly better mileage than the 3.55, but not much because the HP and torque requirements are the exact same. The difference is small because the lower gear will use less throttle to make the same power. In other words, the fuel burn is NOT exponential.

 

Ram57, You are exactly correct in theory. The lower gear will drop out of fuel saver mode less often. It will also drop out of over drive more often....because it has to make more HP because it makes less torque. You guys would love physics, it is nothing but crap like this, just throw in a ton of math.

 

Oh I agree with you that in mountainous/hilly terrain, the 3.92 gears would most likely yield better or equal gas mileage due to it's increased ability of pulling the inclines. That's why in hilly or mountainous areas I turn on the tow/haul mode and allow the extra underdrive gear to work for me. That's explained on pages 308 & 309 of the owners manual.

 

How about another physics problem? Chysler claims a 10% increase in fuel economy with the MDS. If MDS shuts down 50% of the cylinders, why do you not see a 50% increase in economy? Obviously, the system is not on all the time, but if you were driving on a level freeway at 60 mph on cruise and the MDS was on 100% of the time, would your mpg go from 20 to 40? Of course not, it takes "X" horsepower to push your truck through the air at 60 mph. If you drop half your engine, the other half has to make all of the HP. The other cylinders will have to burn more fuel. You also have a large amount of parasitic drag from friction and compression in the dead cylinders.