Gearing Question
Fleet Mechanic
Joined: Oct 2004
Posts: 1,395
Likes: 105
From: Southern KS
In simple terms, the more driveshaft revolutions per axle revolutions you have, the more power is being transferred to the ground.
Gearing is all about torque multiplication. The numerically higher the gear ratio the more torque is being put to the ground. This difference is the very best bang for the buck modification you can do to get seat of the pants improvement.
The trade off is highway cruise will be at a higher rpm. This may or may NOT effect your overall gas mileage. Several factors here are aerodynamics, vehicle weight, tire size etc. For heavy Superdutys, you want a lower rear end (like 4:30's). This will actually help get the heavy mass rolling without your engine working as hard. Also, remember your engine is running at peak efficiency right at the rpm of peak torque. That is where it is getting the most power from each cylinder full of gasoline. This doesn't necessarily mean the best gas mileage is at peak torque rpm, but in a heavy non-aerodynamic vehicle a few extra highway rpms will not hurt gas mileage all that much, if at all. (it may even improve it)
As a bonus, pulling lighter trailers you will be able to pull in overdrive without downshifting. This will extend tranny life and is good for mileage.
Gearing is all about torque multiplication. The numerically higher the gear ratio the more torque is being put to the ground. This difference is the very best bang for the buck modification you can do to get seat of the pants improvement.
The trade off is highway cruise will be at a higher rpm. This may or may NOT effect your overall gas mileage. Several factors here are aerodynamics, vehicle weight, tire size etc. For heavy Superdutys, you want a lower rear end (like 4:30's). This will actually help get the heavy mass rolling without your engine working as hard. Also, remember your engine is running at peak efficiency right at the rpm of peak torque. That is where it is getting the most power from each cylinder full of gasoline. This doesn't necessarily mean the best gas mileage is at peak torque rpm, but in a heavy non-aerodynamic vehicle a few extra highway rpms will not hurt gas mileage all that much, if at all. (it may even improve it)
As a bonus, pulling lighter trailers you will be able to pull in overdrive without downshifting. This will extend tranny life and is good for mileage.
Last edited by super 6.8; Mar 2, 2007 at 05:57 PM.
Sit. Stay.
Joined: Oct 2001
Posts: 18,308
Likes: 20
From: Washington State
Good answer! 
Let me e x p a n d a bit more.
3.73 = the driveshaft will rotate 3.73 times around for each revolution of the tire.
4.10 = well...it'll rotate 4.10 times
...and so on and so forth.
Because your engine is spinning faster and that gas engines make their torque at higher RPM (versus diesel) a lower gearset (higher numerically) will put you closer and keep you in the optimum power production part of the RPM range. (In our case anyway)
Higher gears (lower numerically) are great for long steady-cruise driving. While lower gears (higher numerically) are great for that great low pulling power.
Think of a 10 speed bicycle. The high gears are the ones that are a pain to start out in but will make you haul tail once you get goin'. Your legs just aren't moving real fast to get the speed you want.
The high gears are the one that make you pedal like a banshee just to move 20 feet.
For us, the transmission is kinda like the back wheel of a 10-speed with 5 speeds available. The rear end gear choices is kinda like the front sprockets of a 10 speed. You can choose between a few depending on the mission.
Follow?
Let me e x p a n d a bit more.
3.73 = the driveshaft will rotate 3.73 times around for each revolution of the tire.
4.10 = well...it'll rotate 4.10 times
...and so on and so forth.
Because your engine is spinning faster and that gas engines make their torque at higher RPM (versus diesel) a lower gearset (higher numerically) will put you closer and keep you in the optimum power production part of the RPM range. (In our case anyway)
Higher gears (lower numerically) are great for long steady-cruise driving. While lower gears (higher numerically) are great for that great low pulling power.
Think of a 10 speed bicycle. The high gears are the ones that are a pain to start out in but will make you haul tail once you get goin'. Your legs just aren't moving real fast to get the speed you want.
The high gears are the one that make you pedal like a banshee just to move 20 feet.
For us, the transmission is kinda like the back wheel of a 10-speed with 5 speeds available. The rear end gear choices is kinda like the front sprockets of a 10 speed. You can choose between a few depending on the mission.
Follow?
