If the large diameter tire you put on a Ram pickup
has 'High Rolling Resistance'
and most aggressive tread 'mudder tires' do
then changing the gearing is not going to get you back to the same MPG.

A large diameter tire slightly lifts the truck higher
this lets more air travel underneath the rough underside of the truck
and the drag of the air get worse.

In addition,
a large diameter tire 'sticks out more' from the enclosed fender well
and more of it hits the air,
this FURTHER makes the aerodynamic drag worse.

There are large diameter tires specially built by the tire companys to roll easier.
These are the 19.5 diameter commercial truck tires.
These 19.5 tires and their ability to improve MPG is thoroughly written up in the
Cummins Ram Diesel forums like TDR and DTR

Today with the 125 day supply of new Ram pickups sitting idle at the dealerships
it is easy to find out what differential gear ratios do to highway MPG.

Find a dealership and salesman that will let you and friends/wife/son/brother
'convoy' a few Rams on the same highway at the same speed in the same weather conditions.

Compare 4.56 equipped Power Wagon
to 3.55, 3.73, 3.92 and 4.10 Ram pickups.

If you want to be accurate ballast them to the about the same curb weights by choosing which person/child goes in what Ram.

You are not doing anything that Dodge engineers have not done before.
Out in the Western USA and during winter in Canada
you can see future years Rams traveling in convoys to test them out.
These Rams are equipped with computers and sensors recording thousands of readings
- including MPG - second by second.

 

The next time your on the highway or long stretch of back roads.... set your over head to "inst MPG"... it you don’t have "inst MPG" try resetting your overhead to the Actual MPG prior to a incline/decline/flat ground. Set your SPEED to 2,000RPM and observe the difference in fuel consumption even though the RPM’s have not changed. You will notice that the engine will consume MUCH more fuel on inclines and MUCH less on declines... but the RPM’s have not changed...The only thing that has changed is the LOAD on the vehicle.... So with that said we learn that RPM’s alone do NOT dictate fuel consumption. If RPM’s where solely responsible then the fuel economy would NOT change under load or when the load has been removed and you would get the same fuel economy at each RPM regardless of the load on the engine.

This experiment will help you gain an understanding on what effects fuel economy. Lower the load on the vehicle at any given RPM and your fuel economy will improve…

SPEED SAFE, AIR RAM

 

I wouldn't call using the overhead very scientific. I thought you had some actual engine management knowledge to share.

I'm still not convinced but I do understand why it would happen.

 

Im sory, I think I may have over simplefied it. I never claimed the overhead to be scientific... but it is a good indication as to what to expect when the vehicle is under load or not... Under load you use more fuel... Not under load you use less fuel... Not scientific at all but its clearly "BLACK & WHITE" ...

Let me try again...

YES that is EXACTLY what I was saying.

Yes it does... and these are constant changing variables, I think I may have confused my point when I tried to use the RPM’s alone instead of SPEED/RPM’s together... an example, at 60MPH I'm running 2200 RPM’s...

Your computer controles the A/F ratio through information gathered by several difference sensors... one of them is the throttle positioning sensor..… If you hit a hill going 60MPH@2200RPM and your throttle position is approximately 10% open buy the time you reach the top of that hill your throttle position will have to be open much more (30%-40%) to maintain that the same speed you hit the hill at… Your speed has not changed and as long as your transmission did not down shift your RPM’s would have stayed the same… but that additional throttle allowed more air and fuel in… which was needed to maintain the 60MPH@2200RPM speed/RPM..

When you set the cruz speed to say 60MPH@2200 and start heading up hill... the truck will eventually start to bog down due to the weight of the truck vs. the incline of the hill, the PCM will since this loss in set SPEED and open the throttle % to maintain the set speed. If you where heading down hill it would not requior the same throttle position and would close the throttle % to maintain the set speed… Going up hill you can still maintain the same SPEED/RPM but you need to give it throttle to maintain it. As long as the transmission does not downshift you will maintain the same SPEED/RPM going up this hill... so in OVERDRIVE SPEED and RPM go hand in hand. When the hill becomes too great to overcome with just adjusting the throttle position % the transmission will downshift into a lower gear which will add the needed torque to overcome the hill… but this also causes the RPMs to go through the roof.

A good example how the load effects actual throttle blade % while maintaining the same SPEED/RPM…. …. If you ever set your cruz control and lightly place your foot on the peddle to let it “RIDE/FOLLOW� the peddle movements controlled by the PCM you will notice that although you have set the speed to 60MPH@2200RPM’s you will feel the peddle go down and up in an attempt to maintain the same speed... each time the peddle goes DOWN you use more fuel... each time the peddle goes UP you use less fuel... Its the same principal here.

I hope I was able to help you understand how it knows to add more fuel… Throttle blade positioning would be the answer to your question…

Again I feel I oversimplefied it and inso I never did answer your question "HOW"... But I think above should hold the answer... and that would be throttle blad position...

SPEED SAFE, AIR RAM

 

And what exactly is that going to prove? You commented on gears and MPG but how is comparing an 8000 # (+ or -) 4WD Power Wagon with 4:56 gears to a standard Ram proving the gears are eating up your fuel? If the Power Wagon was not so heavy and demanded so much torque to get going, do you think DC would have put 4:56's in it? They didn't in any other truck. I'm not seeing your point.......


And on a side note, you guys are making this thing waaaaaaayy to complicated.

 

my dad's peeked around the 22 mark a couple times on real long hauls. Right now he's averaging 19. Pure stock (less the tonneau).

 

The powerwagon comes with factory 4.56's.

 

magnethead,

go check the official curb weights on the Ram Power Wagon
better yet, weigh one for yourself
it is not a 8000 lb truck with the 5.7

http://www.trucktrend.com/roadtests/..._overview.html

Note my suggestion to ballast all the trucks in the convoy to about the same weight.

If you use your search button
you'll see that I advocate trying do comparable tests whenever possible,
including swapping two tires between trucks so that the 'average' tire loads are the same too.

If you don't think that a Dodge dealership salesman
wouldn't agree to let you swap tires between Rams,
then you don't realise that A REALY GOOD salesman
wants to learn things too,
including everthing possible about Rams & MPG.

For a salesman, the MPG problem with pickup trucks is a HUGE issue right now and the good ones want to be able to advise not only their current possible customers - they also want to be able to give good advice to PAST customers.

If the salesman who sold your Ram to you 3 years ago
gave you simple tips about how to improve your MPG
and the tips worked,
how would you feel about that salesman
the next time you were shopping for a vehicle?

There are new car salesman who try to trick customers
and there are also salesman who try to help the customers however they can,
including making sure they get good repairs and financing.

Speaking of ballasting trucks,
did you know that if you coast a truck down a steep hill
at least at three different weights
it can tell you things about the truck
(aerodynamics, rolling resistance, drivetrain loss, top speed)
that otherwise would take millions of $ of equipment?

 

If you are buying gears strictly for a possible MPG gain consider this. It will typically take you over 50,000 miles to even recoup a typical gear gear change cost assuming you gain 1 mpg. Also consider that people who do put down the money for any mod are more likley to exagerate the benefits. The CAI and 180 thermostat threads are other prime examples.