Hey everyone,

This has probably been answered but is the reason for regearing the weight of the tires turning or is it just because there is more surface area on the road, causing more friction, making it harder to turn? Thanks for you help its much appreciated

 

more weight further from the center, I am sure friction plays a part too but the weight and distance of that weight from the center is the majority of the problem.

 

I"m sure others will have more detailed explanations but in a nut shell...

Larger tires (increased diameter) turn slower than smaller diameter tires.

At say 70 MPH the tires turn at a certain RPM depending on the diameter. The engine operates most efficiently in a certain range of RPM's. For the 3.8L of our Jeeps this is between 2600 and 3200 RPM.

Gears are about matching the engine RPM's with the tire RPM's.

 

Taller tires are harder to rotate, for two reasons:

1. It takes more torque on the axle to rotate them.
2. The larger diameter means lower engine rpm for a given speed/distance traveled. By definition, lower engine rpm means less power being made.

Regearing increases the torque applied to the axle.
Regearing allows the engine to operate at increased rpm, which means more power is being made.

Another way to look at it:

V6
Automatic in 3rd (1:1)
6 speed in 5th (1:1)
If rear axle = 2:1
You get 6 power strokes per tire revolution.
Change the rear axle to 4:1,
You get 12 power strokes per tire revolution, which feels like a bigger engine. You would go up the hill slower at a given rpm, but you could haul a larger load up that hill.