I have only known GM and Ford gear swaps that changed the gear located in the transmission housing to correct the speedomoeter. I also knew that and that tire size would screw things up as well. Now this is what I found because I just had to know since Dodge does things differently! I just want other people to know these specifics as well because tire sizes and gear swaps are a common thing out there so that way youknow how the gear change and tire swaps effect your speedometer.
This was started in another thread, but I we pretty much hijacked the thread, so I decided to start a new one, not to beat this to death, but to help those who don't understand like I didn't at first because it's a complicated theory to grasp yet very simplistic from the traditional ways.

Here is what AirRam had said about tire changes and gear swaps in another post.

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...

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

Air Ram explained things to an extent, so I feel that this reading is in addition to what he stated. It's short and simple and is easy for most to understand.

All Dodge trucks since 1998 have used an electronic pinion factor. The age of the speedometer drive gear is gone. Today our Rams read the number of revolutions of the tone wheel located in the rear axle housing, reports that to the CAB and in turn to the PCM. Now, some of us want to do some changing of tires, axle ratios, wheels and so on for any number of reasons. Since 1998 the lack of a speedometer drive gear has been a thorn in the side of those who have made wheel, tire, gear changes and learned to recalculate their speed and distance to compensate. For some time the thread in the list was"...how do I correct my speedometer:. ". Aftermarket suppliers turned out corrections that may or may not communicate the correct information to all the systems in the Ram that need the correct speed information. As much as you may not like your dealer or his techs, he has the power to help you out. His diagnostic tools can reach into your Ram, locate the correct values and instill them into the intellegence of the Ram. If you have a TCM replaced, or a CAB, the pinion factor must be reset. If it is correct, chances are you are a happy camper. If not, a host of drivability problems can result.
The dealer's DRB has a range of OEM tire sizes to select, and on Rams, axle ratio may be necessary as well to set pinion factor. The DRB option "other" allows the entry of tire revolutions per mile within the DRB's supported range of selections. Many dealers are using a DART (Diagnostic and Reprogramming Tool) as well as a DRB III. Pinion factor can be set from the DART as well. Your tire supplier can advise you of the number of revs per mile your tire has.

 

Yep, it reads ring gear speed which does not change unless you change tire diameters.

 

Right, and that's what I had a hard time grasping the understanding. There's more to it than that. The sppedo is clocked by the ring gear speed, however, the overall speed is defied by the tire size that has been programmed into the ecu from the factory! That's why tire size makes a diference. It's that little fact that makes everything fall into place!

 

i wana ask a question i think it has been asked before. when using 4.56 gears what will be the top speed of the truck? is it going to be 100 MPH? or more?. thanx