wnjj wrote:
I agree with RCMAN, to a point. Lower gears in the rear end won't make any more power. It will just change the road speed at which your transmission changes gears. That said, it will also decrease the spacing between all of your lower gears which will help with the "torque rise" another poster mentioned. If you're running in 6th gear where you used to run in 5th you now have 5 gears between your current speed and 0 instead of 4. If you don't need to shift on the hill, there will be no difference between a higher rear axle ratio and lower transmission gear versus the opposite.
The reason a gear swap won't matter nearly as much as in the old days is that the rear axle gear swap may move you from 4 to 5 gears to downshift into instead of 2 versus 3 on an old 4-speed. Even with 3.73's, the gear spacing may be close enough already and you just run one gear lower. It's about matching the ratio spread to the power curve of the engine which is why you wouldn't want 2.20's and 2nd gear.
If you look at the total gear ratio to the rear wheels the % spread between gears is the same regardless of the rear end ratio. Thus the RPM change will be the same.
Ford Total 3.73 4.3 Total
6R140 3.73 % Change %Change 4.3
3.97 14.8081 17.071
2.32 8.6536 58.43828715 58.43828715 9.976
1.52 5.6696 65.51724138 65.51724138 6.536
1.15 4.2895 75.65789474 75.65789474 4.945
0.86 3.2078 74.7826087 74.7826087 3.698
0.67 2.4991 77.90697674 77.90697674 2.881
There is no magic in this just basic physics. Gearing does not change the amount of horsepower an engine can produce. The OP can spend $5000 on gears only to find out the 4.3 gears will not enable him to go up a mountain any faster than with 3.73 provided he lets the transmission do its job of picking the optimum gear for the task.
The only time he will have a big difference is in first gear. After that both ratios will behave the same as he goes through the gears until he hits 6th. Then the 3.73 will allow a greater top speed which with both gears will be much faster than we should be driving a pickup.
And if you had an engine that produced its max horsepower at 900 rpm then you would probably want a 2.20 rear end if the transmission had the same ratios as the 6R140.
