3:31 Gearing

Huntindog wrote:

rjstractor wrote:

Huntindog wrote:

LIKE2BUILD wrote:

cliffy49 wrote:
Looking into....a 2012 F250 with a 6.7 and 3:31 gears.

I realize that 3:31s are not normally the ideal choice for towing but since this is a diesel, it may be different. I realize that 4:10s or 3:73s would be a better choice.

In previous truck generations when 3 or 4 speed transmissions were your only options, lowering the axle gear ratios was the only good way to increase applied torque to the wheels.

Newer transmissions with 6, 8, and 10 gears let the manufacturers reduce axle ratio for fuel efficiency and allows for using lower ratios on gears 1 & 2 to give you 'off the line' grunt even with a higher axle ratio. So, diesel or gas, the increased # of gears in the transmission makes up for the higher axle ratios.

KJ

Ummm, the truck in question does not have 8 or 10 gears. It has 6. Two of which are overdrives.
So in effect it is similar to the old 3-4 speeds you are talking about... A double OD tranny will work really well with lower gears.
3.31s,,,,not so much.

I can't understand why guys are so fixated on just the rear axle ratio without considering transmission gearing. But the engineers who design the trucks do. The Ford 6R140s ratios are lower across all gears except reverse compared to an Allison 1000. The first 3 gears of the Ford with 3.31s are close to the Allison with 4.10s, and the top 3 roughly the equivalent of the Allison with 3.73s, and I think everyone agrees that the Duramax pulls well with those ratios.

You are forgetting the effect of tire size on gear ratio. The Duramax does NOT come with 4.10s. But most (maybe all) all have smaller tire diameters than the Fords.
You cannot just fixate on gears.

Transmission gears come into play as do final drive ratios and tire size but in addition to that is the year of the engine. The older trucks only made 765 lb ft of torque or less while the new trucks are putting out up to 940 lb ft.