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So, I'm going to duplicate some math here, and it's to help me wrap my head around everything.
My Insight:
FD - 3.208
1st gear - 3.266
5th gear - 0.710
Tires - 165/65r14
Weight - 1850#
Torque - 67ft-lbs
Power to weight - 72ft-lbs/ton
Your truck:
Current FD - 3.73
Proposed FD - 3.45 and 3.27
1st gear - 3.72
5th gear - 0.79
Tires - 235/75r15
Weight - 3100#
Torque - 154ft-lbs
Power to weight - 99ft-lbs/ton
Comparatively speaking,
With your 3.73 FD, your RPM in 1st and 5th gear are almost identical to my Insight's. You have 42% better power to weight in 1st gear, and 37.5% in 5th gear, taking tire size into account.
With a 3.45 FD, you'd have 31% better power to weight in 1st gear, and 26% better power to weight in 5th.
With a 3.27 FD, you'd have 25% better power to weight in 1st gear, and 20% better power to weight in 5th.
Taking tire size into account, to have the same power to weight in 1st gear (and thus effectively equally "tall" gearing) you'd need 2.63 final drive. To have the same power to weight in 5th gear, you'd need a 2.71 final drive.
Or, in other words, even with a 3.27 FD and bigger tires you'd still have considerably more power in every gear than I do. I'll admit that I could do with a slightly shorter first gear without the IMA to help, but it's very drivable.
Totally up to you just how tall you go.
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