So, I believe I have 3.96 stock gears. Will switching to 4.56 help my gas mileage or make it worse. I drive mostly short distance but ensure I do a nice long drive about every other tank. If it will help MPG, what gear manufacturer do you recommend? Is it an install I can do myself? I have changed the spider gear in a jeep before, same thing? Thanks ahead of time for the input.

 

Look on the glove box door to be sure of the ratio. If not there, look for a tag on the diff. Shorter gears will use more gas, but with the oversize tires, it'll help off the line, getting the weight moving easier. Highway rpm will be higher, using more gas. Those thirty-fives have made your truck a bit more long-legged than stock. The 3.92 is probably down to 3.7 or so. The 4.56 will be more like 4.10 or so. Probably a decent exhange -- trade a bit of mileage for better jump.

 

I am looking for better MPG, but, I don't want to be screwed when I go mudding you know?

 

I would think you'll definitely need the extra torque for any kind of off-roading.

 

The change to 4.56 from 3.92's will provide a nice performance boost, without a doubt, especially with larger than stock tires.
For me, personally, with 35" tires the added performance (acceleration) would be worth it even if I experienced a moderate loss in MPG's. This however, was not the case, and IS not the case in the vast majority who have gone to them.
I found that I gained between 1 & 2 mpg's in normal around town driving, as the truck accelerates from a dead stop with much less throttle. My mileage on the open road is relational to the speed I'm driving. On country roads, where the speed limit is from say 45 to 60 my mileage is relatively the same and on the interstates my mileage goes from just slightly worse than before at say 65 to a mile or two per gallon worse as my cruising speed goes through the 70s to around 80 mph.

 

Our 3rd Gen Rams are OVER DRIVEN from the factory.

If your tire hight is 30"+ tall you will in fact see a GAIN in MPG up to 65-70MPH going up to a 4.56 gear ratio.

I went from 3.55's to 4.56's and gained 3-4 MPG@60MPH@2200RPMs ... Not an opinon, this is a FACT!

So I gained 300RPM's at 60MPH and 3-4MPG at that same speed.

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The math:
45RFE .75 OD (Pre-2002)
545RFE .67 OD (2003-UP)

Find your overall Gear Ratio:
REAR GEAR x TRANSMISSION GEAR = OVERALL GEAR RATIO (OGR)

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Find your RPMS at ANY speed useing the OGR:
SPEED x OGR x 336 / TIRE HIGHT = RPMs

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My personal rule of thumb for best overall performance and fuel economey, aim to gear to acheive 2200RPMs@60MPH .... Chose a gear that lands you there and you can have your cake and eat it too!


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Now one of the biggest mistakes people comonly make is to think that RPMs alone dictate the fuel consumption... This is FAR from the truth.

Its the load on the engine at any given RPM that has more to do with fuel consumption then RPMs alone.

Do a little test (For those with an overhead computer with Inst MPG reading):

Get on the highway and Set the cruz to 2000RPMs ..... Watch the over head MPG numbers. You will notice that on a Flat Ground you will see 18-19MPG (EXAMPLE) but when you hit an incline it drops to 11-12MPG... then a decline shows 28-30MPG... the entire time the RPMS NEVER went above 2000RPMS. This proves that the load on the engine has a greater effect on MPG then the RPMs alone!

SPEED SAFE, AIR RAM

 

don't do the gear change with increased mileage as your sole reason. it'll cost around $1k for a shop to get everything done. and how much gas can you buy for that. how long before the gears pay for themselves. i'd do the gears to help w/ larger tires or for towing...or just because you want to...but not for fuel savings alone. now if you're using it (mpg's) as an excuse so the wife will allow it...then absolutely, you'll probably go up to 25mpg's city and 40-ish hwy

 

yea air i did a test myself the other day and @ 1800 rpm's i was gettin about 14-16 mpg around 2000rpm's i was gettin about 17 and @ or around 2200 rpm's i was gettin 20mpg on flat ground.

 

Thanks all for the info. I believe this is something I will look into. Now the second part of the question, is this something I can do myself? I have changed out spider gears in a Jeep, but that is about the extent of my gear knowledge.

 

NO, if you do not have specific experiecne with the Dodge 9.25 rear then you should let an experienced shop do the work for you.

I have personaly built engines/transmissions/suspensions but I have yet to sum up the currage to do gears... Not that I dont think I can do it... but more so that I cant afford to learn the hard way just yet. If I cant afford to do it all over if it goes bad... then I will let somone else do it. Gear Jobs gone bad are expensive.

SPEED SAFE, AIR RAM