Hammer is right on this one. Everyone thinks that the slower an engine turns the less fuel it burns. Every engine i have built and helped to build has a speed at which that particular engine is most effiecient. Had an 91 olds cutlass with the quad-4, mileage at 65 mph was 30, at 75 mph mileage was 36, at 83 mph on freeway, mileage was 41. I know it sounds crazy but i owned the car for 5 years and the mileage was the same the whole time i had it. Every engine has a 'SWEET SPOT'.
Thanks guys

 

+1

I went with 4:56 and gained 1-2 MPG city and probably lost 1 MPG highway.

So With Out a Doubt I would go with 4:56. If you have a 4wheel parts near you, then I would go to them for the gears! When I was shopping for gears I did some price shopping and they price matched! I also had them install a Detroit Tru Trac LSD for next to nothing after some negotiations. So you might want to see if you can have that done at the same time since they will already be in the rear Diff.


Get several prices and then bring them proof of that price and ask them to match.

www.4wheelparts.com

 

I was infact saying that taller tires effect speedo calibration as well as the gear sizes! I potrayed the "question" as Air Ram spoke it. It was an act of mocking if you will. I suppose the table is turned on me now because I couldn't possibly fathom the idea why a tire size changes the speedo and that the gear ratio doesn't. They are on the same driveline and interact with one another! In short words, the ring can only turn as fast as the tire allows it! They are in other words locked together on the same shaft. It's the tire size that ultimately make the final drive ratio in reality. Please divulge some more info so it will help me understand. Thanks



Sorry to "hijack" your thread, but it's technically proper since you are changing both of these components out.
Also, the Detroit TruTrac is awesome. It's a worm drive and works instantly with no bind or chatter! Highly reccomended. No friction pack to burn out and no special limited slip fluid needed.

 

In order to grasp the concept you need to know what a RING GEAR IS. The RING GEAR is bolted to your CARRIER. Your CARRIER turns ONE revolution each time the TIRE turns ONE revolution. This means the RING GEAR also turns ONE revolution for each full revolution of the tire. So if the SPEED sensor is taking the reading from the RING GEAR then it does NOT matter what the gear ratio is... because ONE revolution of the tire will ALWAYS be ONE revolution at the RING GEAR. If you have 3.26 gears the ring gear will only turn ONE REVOLUTION per the tires... If you have 5.86 gears the ring gear will only turn ONE REVOLUTION per the tires... so gear ratio has ZERO effect on the CHRYSLER 9.25 SPEED READINGS.

NOW when your tire hight changes the distance the tire covers per revolution changes with it.

So If ONE revolution = 5 feet on a tall tire but you put a shorter tire and it covers 4 feet you will effectively be moving SLOWER but the SPEED sensor would read FASTER... Because ONE REVOLUTION is ALWAYS one REVOLUTION.

I hope that helped... It may have confused things further... if so let me know and I will take another shot at it.

NOW back to GEARS...

Contrary to popular Misconception... RPM's alone do NOT dictate fuel economy.

Its the LOAD on the engine at any given RPM has a greater effect on MPG then RPM's alone.

Do a little test. If you have an overhead computer, set it to INSTANT MPG setting.

- Set the cruz to 2,000 RPMs and leave it. Take note on FLAT ground the MPG will read 18-19MPG but as soon as you hit an incline the MPG drop to 13-14MPG... And then if you go down hill the MPG goes up to 28-30MPG... Yet the entire time the RPM's have not changed... the only thing that changed was the load on the engine.

With 4.56 gears and a .67 overdrive you effectively create a 3.05 overall gear ratio!
(REAR GEAR x TRANS GEAR = OVERALL GEAR RATIO) 3.05 gears are still considered GRANNY GEARS!

So what you get is literally both an improvment in the CITY & HIGHWAY since it takes less load to get your 5,000LB truck moving from a stop... and less to keep it moving at speed.

The 4.56's teamed up with a 32" tall tire will show in most cases an IMPROVEMENT in MPG from 0-70MPH... but even at 75-80MPH it will not be bad and may be equal to what you where seeing prior to the gears...

DO THE MATH!

1) Find the overall gear ratio:
REAR GEAR x TRANS GEAR = OVERALL GEAR RATIO (OGR)

2) Find your RPM's at any speed using the OGR:
SPEED x OGR x 336 / TIRE HIGHT = RPM


For best overall performance I recomend to chose a rear gear that will put your RPMs at 2100 RPM's@60MPH. Fight the desire to chose a gear that puts the RPM's BELOW 2000RPMs@60MPH... thats defeating the purpose of the gear swap and the results will NOT be as rewarding both in MPG and performance.
If there are ANY questions please feel free to contact me.


SPEED SAFE, AIR RAM

 

I know what the friggin ring gear is. I am a certified mechanic! I have changed many gear before, just never on a Dodge! Now as you said, the ring gear is bolted to the carrier. Yes we all know that. NOW, if your ring gear rotates ONE revoulution, that means your TIRES have rotated one revolution REDGARDLESS of tire size! You can put 80" tires on there and one reveolution is still one revolution of the carrier/ring gear!
There must be more to this than what you are tryin to describe?

 

I guess I just don't understand exactly it is YOU don't understand.

I thought your question was something to this effect:

"Why changing the gear ratio on a Chrysler 9.25 rear axle does NOT effect the speedometer a single bit?"

I'm sorry if it sounds like I'm attempting to break it down too low of a level... Im not trying to belittle you.... I'm just trying to help you understand... Your the one who does NOT understand how it works and your the one who is asking the questions... so try not to get so overly defensive/offensive when people try to help you learn something that apparantly you don't understand!

Traditionally the speed sensors are located on the transmission tail housing...It counts the rotations of the Driveshaft. And before you ATTACK... I understand your a "CERTIFIED" Mechanic and you all ready knew that and everything else I'm about to share with you...

In that case a 4.56 gear change would effect the speed reading since ONE full revolution of the tire/ring gear would equate to 4.56 rotations of the drive shaft.... So if your PCM is set up to do the math with 3.55 gears and your sensor is reading the rotation of your drive shaft then you swap in some 4.56's your speedo will be off since the drive shaft rotations will directly relate to the pinion gear/ratio. But since Mother MOPAR decided to make it simple they moved the speed sensor to the Axle housing...

So instead of counting the multiple rotations of the drive shaft... its counting the rotations of the ring gear...

In a gear set the only part that changes in the amount of rotations would be the PINION gear to the drive shaft to the transmission...

So if your taking a speed reading anywhere along that line your speedo would be effected by a gear swap.

But if your sensor is reading the ring gear rotations then the speedo will NOT be effected by a gear swap...


SPEED SAFE, AIR RAM

 

VERY GOOD explanation Air Ram, not only did it cover everything but more importantly, you saved me from having to write a "novel"...

 

Im just trying to help everyone understand how a gear swap from 3.55's to 4.56's can literally improve ALL ROUND PERFORMANCE. This includes an INCREASE in fuel economy from 0MPH up to 65-70MPH. Its all about MATH... And a great thing about it, you don't need to adjust for it, this is why when you purchase a SUPERCHIPS programmer.... they only offer the ability to adjust for tire height. There programmers will not adjust for gear ratio... because its not an option.

If anyone has any questions on gears please feel free to contact me... I have been dealing with the 3rd Gen Ram since 2002 and have been dealing with gearing our trucks just as long... I can get you some of the best prices SHIPPED FREE to your door Coast to Coast...

SPEED SAFE, AIR RAM

 

Thanks AIR RAM for the Info. I have two more Questions.

Does the carrier have adjusting nuts or Shims pack?

Where is the best place the get the gear set from?

Found this place?
http://nationaldrivetrain.com/shopsi...RYSLER925.HTML

Thanks everyone for your input.