Need Gear ratio advice
Veteran
Joined: Dec 2010
Posts: 422
Likes: 0
From: North Aurora, Il - 40 miles west of Chicago
Well, 3.21 arent that good. They use too high rpm to get goin. I do 36 miles round trip to work about half city half highway. Im only gettin 15mpg. I can get 20 highway if i drive to kansas city and not stop. Of course it doesnt help in Kansas that there is always a 30mph wind !
So, i dont think the 3.92 is goin to hurt my mileage much. But i dont want to get 4.10s if its going to have a jerky too much torque feel and 11 mpg.
Just trying to get some good advice. Thanks
So, i dont think the 3.92 is goin to hurt my mileage much. But i dont want to get 4.10s if its going to have a jerky too much torque feel and 11 mpg.
Just trying to get some good advice. Thanks
I do hundreds and hundreds of diffs.
Your wasting money going from 3.2 to 3.9 with a 5k truck.
I would recommend 4.10 or my 1st choice 4.56.
You mpg isn't going to sink to 11 mpg unless you regulary cruise at 100 mph.
Why exactly do you want to change gears ?
Your wasting money going from 3.2 to 3.9 with a 5k truck.
I would recommend 4.10 or my 1st choice 4.56.
You mpg isn't going to sink to 11 mpg unless you regulary cruise at 100 mph.
Why exactly do you want to change gears ?
Administrator
Joined: Jul 2007
Posts: 24,686
Likes: 21
From: South Georgia/East Florida
As I stated above, IMO 4.10:1 is the PERFECT gearing for a balance of power and economy.
Does this mean that 4.10 gears are what you should purchase? Maybe/Maybe Not...
The important thing to look at when thinking about gears is not what the physical gear ratio is, but rather what your EFFECTIVE GEAR RATIO will be.
Your truck's gear ratio is figured at base wheel diameter, what that base (stock) diameter is, will be on your door sticker (you may need to convert metric to inches).
Tire size dramatically changes your effective ratio. Smaller diameter tires will raise this ratio and larger tires will drop it.
To calculate your actual gear ratio, the following formula should be used:
Effective Gear Ratio = stock tire diameter / new tire diameter x physical gear ratio
So, using the above, if you went from say a stock 31.5" tire and your stock gears are 3.92s, your effective gear ratio when going to 35" tires would be approx. 3.5 : 1. A significant difference. If your stock gears are 3.55s then the effective ratio after the tire change would be 3.19 :1 and if you have the 3.21 gears you are now at a pathetic 2.88 : 1.
A change to 4.56 gears with 35" tires would have your effective ratio at precisely 4.10 : 1, which IMO is THE perfect gearing for a 5000 lb. plus truck with a 545rfe transmission utilizing a 4th gear ratio of 0.75 :1 and a 5th gear ratio of 0.67 : 1...
At this ratio, your truck will get up into the powerband faster from a dead stop, requiring less throttle pressure (saving gas), while at the same time not be taching too high while on the interstate. Also when passing on highways, because you will be cruising at a slightly higher rpm, there will be no "lag" when hitting the gas, a constant complaint from 4.7 and Hemi owners...
Does this mean that 4.10 gears are what you should purchase? Maybe/Maybe Not...
The important thing to look at when thinking about gears is not what the physical gear ratio is, but rather what your EFFECTIVE GEAR RATIO will be.
Your truck's gear ratio is figured at base wheel diameter, what that base (stock) diameter is, will be on your door sticker (you may need to convert metric to inches).
Tire size dramatically changes your effective ratio. Smaller diameter tires will raise this ratio and larger tires will drop it.
To calculate your actual gear ratio, the following formula should be used:
Effective Gear Ratio = stock tire diameter / new tire diameter x physical gear ratio
So, using the above, if you went from say a stock 31.5" tire and your stock gears are 3.92s, your effective gear ratio when going to 35" tires would be approx. 3.5 : 1. A significant difference. If your stock gears are 3.55s then the effective ratio after the tire change would be 3.19 :1 and if you have the 3.21 gears you are now at a pathetic 2.88 : 1.
A change to 4.56 gears with 35" tires would have your effective ratio at precisely 4.10 : 1, which IMO is THE perfect gearing for a 5000 lb. plus truck with a 545rfe transmission utilizing a 4th gear ratio of 0.75 :1 and a 5th gear ratio of 0.67 : 1...
At this ratio, your truck will get up into the powerband faster from a dead stop, requiring less throttle pressure (saving gas), while at the same time not be taching too high while on the interstate. Also when passing on highways, because you will be cruising at a slightly higher rpm, there will be no "lag" when hitting the gas, a constant complaint from 4.7 and Hemi owners...
Last edited by HammerZ71; Apr 24, 2011 at 10:45 AM.
Professional
Joined: Feb 2006
Posts: 242
Likes: 0
From: America
well I myself would opt for 3.92's at least with the stock ratio it would be harder for dodge to claim that your rear end caused the tranny to go out. not saying they would not try but harder to claim if you went with a ratio that was offered from the factory, now if your not worried about that, and you can afford the extra gas and want the extra ummp then go 4.10's
