I have been reading your posts about gear ratios with interest. I know from my own experience with airplanes that manifold pressure has more to do with fuel consumption than does the rpm. Also on my diesel tractors, when you are pulling equipment that is a light load on the engine, the fuel burn per hour is much less than pulling equipment with a heavy draft at the same rpm. So it makes since to me that the same thing would be true with a truck. However, there must be a different transmission ratio in a 2006 Hemi Ram than my 2002 Ram. Both have the 3.55 axle ratio but the 02 turns 1600 at 60 and the 06 with a 5 speed auto turns about 1725. I guess my question is this: The Hemi has MDS. Would reducing the load on the engine by turning another 300 rpm allow the Hemi MDS to run on 4 cylinders at a higher highway speed or would the MDS cut out sooner because of the extra 300 rpm.
Comments from everyone is appreciated.
Thanks

 

jimaire,
I hope AirRam tries to answer your question
(just for the entertainment factor)
but with your airplane engine and diesel tractor experience
you already know more.

You can answer your own question with a smaller diameter tire experiment.
You don't have to spend the money on diff gears.
Look around for a friend/neighbor with low profile tires you could 'borrow' for a short time,
or consider buying some cheap used tires at a salvage yard.
Even new tire makers like Bridgestone have 30 day 'no questions asked' return policies.

Your question can be re-phrased to 'airplane like' statements like:

"Can an airplane travelling
at a constant airspeed and altitude
with a spark ignited engine
improve its fuel economy by
increasing the rpm and lowering the manifold pressure?"

and

"Is the fuel saving from switching from 8 cylinder operation at 30-40% throttle to 4 cylinder operation at 70-90% throttle great enough to justify generating more engine friction with higher rpm?"

Ask older pilots this question - especially those of the World War II age where their fuel economy trainning was a life or death matter.

Your question is not that different than asking a pilot of a two engine bomber in Col Jimmy Dolittle's raid on Japan:

"Could you have travelled further into China during your escape with the little fuel you had left if you had feathered the prop on one engine and then slightly increased the rpm of the remaining engine's prop to keep the same airspeed?"

Even 60 years ago Jimmy Dolittle could have answered this question
because he did extensive experiments in Florida before the raid.

Jimmy Dolittle was not just a brave soldier
he was a winning airplane racer
engine souper-upper
with a PhD from MIT.
Courage and Brains in one package.

Note that I did not say
"Shut off the second airplane engine"
because on Hemi 8/4 MDS the other 4 cylinders continue to pump the pistons up and down.

Why not mount a vacuum/map gauge on your Hemi
so that you can get manifold pressure just like on an airplane?

 

Jimaire, Personally I do not have any experience with the MDS so I’m not sure how it will react. The experience I share is what I have learned with my 2002 3rd Gen Ram with the 45RFE & 545RFE with the .75OD & .75/.67OD respectfully.

With my PERSONAL experience, my 3.55 gears would get a best of 19MPG@65MPH with a tail wind.... after getting the 4.56's installed I can get over 20MPG@65MPH with a HEADWIND! So the gains are obvious and they go hand in hand with your experience with your tractors. I have never flown a plane but it sounds like similar results.

If I had to make an educated guess (Hank you can’t make those due to LACK of personal experience with the 3rd Gen Ram) I would say that MDS would have only 4 cylinders moving the truck at a given highway speed that they would also benefit from the steeper gears… I honestly believe (From personal experience) that it’s the combination of tire height (30�+) and our overdrives (.75/.67) that make a steep gear necessary. So since the MDS is essentially cutting the available POWER in half I would think the 4 operating cylinders would welcome the steeper gears and the additional 300RPMs they bring with them to become more efficient. But until someone with MDS takes the jump to a 4.56 gear we can only speculate at the results… Kind of like Hank does in all his post, “Speculates�.


Unlike Hank, I will not offer advice on a subject I have no "PERSONAL" experience with. Hank will have you thinking he is all knowing and offer his advice even though he really has NO experience on the subject… and when you disagree with him he will attempt to make you look bad.

Understand right away that Hank has social issues and I would not be surprised if he takes meds for those issues. He try’s to offer advice on subjects he has ZERO experience in (Hank all your 1rst Gen Ram experience does NOT help us 3rd Gen Ram owners) ... so take his advice with a grain of salt... sometimes he has SOME (I stress SOME) information that’s useful but that’s only when he "Cuts & PASTE" from existing internet articles. When he uses his own words his lack of experience shines through. The only personal experience he has is with "LIP JACKING" and belittling members. Just look up half of he post and you will see the "CUT & PASTE" method he uses... often his post rarely address the topic/subject... and that’s because he lacks personal experience. You will notice him attack me every chance he gets... I suppose it’s because I call him out on his BS and I don’t let him get away with misleading members. I will say that if Hank was on a 1st Gen Ram forum... his advice may actually be worth something... but because he offers advice to the 3rd Gen Ram owner without ever driving one the value of his advice is borderline worthless.

Shoot away hank... I know you want too. But be advised I will not respond to your BS unless it’s completely out of line... which knowing you your post will be... But I will try my best to ignore you like half the other members.

SPEED SAFE, AIR RAM

 

PASTE has an "E" at the end...

 

LOL.. Good catch.

SPEED SAFE, AIR RAM

 

Good Call!

 

I just corrected it... I was kind of in a hurry when I typed it. I’m not the best typer nor speller in the world... But I normally can get my point across.

SPEED SAFE, AIR RAM

 

=)

You guys are awesome. Thanks for all the informative responses to these types of questions.

 

this could be a quick answer to your question. the 06 power wagon's have the 4.56 stock. they also come with MDS. therefore, there must not be any problem with the 2 combined. mineis the 06 1500 4x4 trx off-road package, and with the 3.92's, i can get up to 22 mpg on the freeway with cruise on. i only got 1300 miles though. the gas mileage isn't supposed to peak out until 6k miles...or so i've heard. i think with the gears, and air flow mods that improve mileage, these trucks with the MDS could possibly see 24mpg on the freeway. that ain't too bad for a truck. of course, it's hard to compare mileage when pretty much every truck is different, whether it be gearing, 4x4 or 4x2, RC, QC, or MC, or a truck on the 2500 platform. i do know however that the hemi's are still going to get better mileage than the CTD trucks unless you're towing. even then though, it's only like 2 mpg less. dunno if i've answered anything, or if i just rambled on. lol. o well. that's what forums are for.