3:31 Gearing

4x4ord wrote:

dodge guy wrote:

4x4ord wrote:

Lynnmor wrote:
Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

If you read and understand my explanation above you will see that a 3.31 with 18" wheels can certainly out pull 3.55 gears and 20" wheels. The scenario above would exist with about a 15000 lb trailer going up a 4% grade. The same sort of scenario would take place in third gear with a 15000 lb trailer on about a 6-7% grade.

So on a 7% grade the trucks will drop into 3rd gear with 15000 lb trailers in tow. The 3.31 geared truck would rev to 2900 rpm and be travelling 55 mph..... the 3.55 geared truck would be travelling 52 mph at 2900 rpm.

You never mentioned 2 different wheel sizes until now. Of course they would tow the same, the final drive ends up almost identical. Now take the same size wheel and 2 different gear ratios and the lower geared one will out perform the higher geared one.

With the same size wheel and two different axle ratio the same scenarios can occur. The idea is this: A truck makes optimum use of its engine's HP when the gearing perfectly matches the load such that the engine is 100% loaded at the rpm where peak HP is obtained.

So consider two identical trucks other than rear gears.... both are 18" tires.

Truck 1 has 3.31 gears. At 2900 rpm it travels:
75 mph in 4th gear
57 mph in 3rd gear

The truck with 3.55 gears will travel:
70 mph in 4th gear
53 mph in 3rd gear

So if the hill incline/weight of trailer require full power in the range of about 55 to 63 mph or about 74 to 77 mph the 3.31 gear ratio has the advantage.

If the speed/weight combination are such that full power is required at 50 to 54 mph or 65 to 73 mph the 3.55 ratio will have the advantage.