Hey fellas, i was wondering if anyone has chagned their gears from the 3.92 stock to 4.56?
I i am thinking of getting some gears, but i am affraid of the mileage in the highway, i have 3.92 , 20's and stock 33'.
With 4.56 you should get better mileage in city driving but worse in highway, right. [sm=confused06.gif]
If anyone has had this change, some feed back would be appreciated.

 

My Power Wagon has 4.56's with 33's and I'm averaging 11.5-12.0 around town and about 14.5 on the highway for short trips(less than 200 miles). I did have a one time best of 16.2 on the highway, but I haven't taken any long trips yet.

 

City driving is dominated by how the driver behaves, like
how fast you accelerate (3-6 MPG) ,
how much you brake instead of coast('negative' MPG),
and how much you are stuck idling ( 0 MPG).

4.56 in the city will usually get slightly worse MPG, but since some driver's are after 'thrills' the extra noise and vibrations of 4.56 gears can actually fool them into slowing down a bit, which will increase MPG.

4.56 will almost always hurt MPG at steady highway cruise in the 50-80 mph range. Above 85 or so it won't matter.

You can verify this for yourself by finding a fellow Ram owner who already has 4.10, 4.56 or 4.88 diff gears and following him on the highway while talking with cellphones or walkie-talkies about what the trip computers say after being reset at the same time. This way you are sure you are at the same speed, same pavement, same wind - and there is not the confusion of inaccurate speedometers because you can ask what the other's speedo is reading too. If you want to check speedos, mileposts should go by every 60 seconds at 60 mph....and every 50 seconds at 72 mph.

If you really want to be accurate you can also swap two tires between each truck so that the 'average' tires are exactly the same.

If you don't do a careful test like this, asking for MPG's is like asking how big that fish was.....

4.56 will affect 5.7 and 5.9V8s more than 4.7V8 and even less on the 3.9/3.7 V6s. The higher gears are more MPG neutral for smaller displacement engines.

 

well, this I have an input on.....
My toy has 456'sand I travel over the road about once every 3 months, usually logging about 750 miles a day for 2 days.......

So this is what I have found...
First of all Hank is right regarding the thrill issue, im 58 and the thrill is gone, so mileage is better, LOL...
secondly my last trip, I cruised at 70 to 75, kept it constant, and found 16.5mpg, ( used the fill the tank method, along with the tripometer).......plus I kept my foot out of the engine, lol.......was constant, and at the time it was JUST after Katrina, so gas prices were to the moon....

And its true, if you drive like your nuts are on fire, you will get crap with the hemi, I do that too sometimes ...in fact just got a superchip, and the first 3 days, smoked my 12" sneakers, and got about 8mpg......but had a ball messin' around.

Its obvious that your mileage will change, but so will your driving habits, you'll want to put your foot in it for a while, but that will fade with time.........also you become once you drive a hemi for a while, and it gets second nature knowing you have that power under you
so your foot gets lazy........

Ok, I talk too much, sorry...just drive, enjoy and be safe....................later, GUIDO



[sm=americanasmiley.gif]
Nam-Vet

 

Thanks for posting that guido7.

I have been filling in the specs for 3rd gen 5.7Hemi Rams
in a software program called 'Fuel Economy Calculator'
and was interested in what it would say
with 4.56 gears versus the standard diffs of 3.55 or 3.73

At 71 mph the program predicted 16.57 MPG at 2306 rpm
which matches your experience
and estimated that the engine was at 52% load
and had 48% of maximum torque in reserve for passing or climbing hills.
With this 48% reserve torque the engine could take the truck up a 5.7% grade hill
(which is fairly steep) without downshifting.

With 3.92 diff gears the program predicts
at 71 mph the MPG would improve to 17.07 at 1983 rpm
and estimated the engine was at 61% load.
With 39% reserve torque the engine could take the truck up a 3.9% grade hill
without downshifting

With 3.73 diff gears the program predicts
at 71 mph the MPG would improve to 17.16 at 1883 rpm
and the estimated engine load was 65%
With 35% reserve torque the engine could take the truck up a 3.2% hill
without downshifting.

With 3.55 diff gears the program predicts
at 71 mph the MPG would improve to 17.22 at 1796 rpm
and the estimated engine load was 70%
With 30% reserve torque the engine could take the truck up a 2.6% hill
without downshifting (most gentle interstate hills are less than 2%)

With 3.21 diff gears the program predicts
at 71 mph the MPG would decrease to 16.19 at 1624 rpm
and the estimated engine load was 80%
which is into the 'fuel enrichment mode' to prevent pinging
where the air to fuel ratio is richened from normal 14.7 down to 11
With 20% reserve torque the engine could take the truck up a 1.5% grade
without downshifting.

This is not to say that 3.21 diff gears cannot improve MPG
under the right conditions.

For example the 3rd Gen Rams (Cd=0.528 and 35.1 sq feet) are
quite a bit blunter and less aerodynamic
than the 2nd Gen Rams (Cd=0.44 and 34.49 sq feet).
If the aerodynamics of the new Rams could be made as good as the
older 1994-2001 Rams then:

The program predicts with 3.21 diff gears, 5.7 Hemi and 1994 aero specs
at 71 mph the MPG would be 19.51 at 1624 rpm
and the engine load would be 70%
and the engine could pull the better aero truck up a 2.3% grade without downshifting.

DaimlerChrysler recently showed off a high MPG show car called the 'BoxFish'
that imitated the shape of a fish to get the a 'realworld' Cd of 0.19
{the real fish is even better at Cd=0.10}

If a 3rd Gen Ram had the aerodynamic Cd of the BoxFish show car
and its present 35.1 square feet of frontal area
the program predicts
at 71 mph with 2.73 diff gears the MPG would be 28.55 at 1381 rpm
and the engine load would be 57%
with 43% reserve torque at this low rpm
the engine could pull a 2.5% grade hill with downshifting.

 

I went from 392 to 456 in my 04, QC, 4X4. Mpg went up around 1.5mpg in town.

But I'm a lightfoot. Take it easy from a stop.

Stay under 65 and your mileage will increase enough to pay for the changeover in awhile.

 

Where did you guys get your gears and what was the average cost? This is something I'm considering doing.

 

You asked the wrong question; forget miles per gallon and ask gallons per mile!

btw, I too am 58 and the desire to put my foot in it hasn't, doesn't fade.

 

I have an 02 QC 4x4 5.9L with 35s. When I put my 456s in my MPG took a nose dive. I usually set the cruise at 65. To get the same MPG I have to be at 55. I will be putting 37s on shortly and according to the gearing chart should put me close to stock gearing. So, hopefully I will be able to go a little faster on the hwy to stay in the 13-15 mpg range.

 

Bringing back an old thread, thanks search function...

Have you went to 37's yet? If so how was your mileage affected? I just bought a used '06 w/37's and am planning a regear. calculations call for around 4.28 to return to stock ratios but we only have 4.10 and 4.56 offerings. I am leaning to the 4.56 for future towing but feel the daily economy payoff may be better with the 4.10's. I also plan on going down to 35's in the future once the 37's are worn or traded, figure this would reduce weight on the drive train some and still fill the wheel well gap pretty well. Thoughts?