snip...
Transmission
ZF 8HP45, 8 speeds
1st, 4.71:1 -- exceptionally low
2nd, 3.14:1
3rd, 2.10:1
4th, 1.67:1
5th, 1.29:1
6th, 1.00:1
7th, 0.84:1
8th, 0.67:1
snip...
Many good, educated, comments in this thread guys, a classic, healthy portal discussion :victory:
Nice to see those numbers posted here Chip. You're correct, exceptionally low for an automatic. The 3.5:1-ish first gears in my Toyota automatics are/were considered very low, but
4.7:1 would be a normal first gear for a manual tranny. Starting a load is important, and so is keeping it going...the tight spacing of those 8 gears should help. I like that the top gear is not overly tall, as long as one specs a low enough axle ratio. The 6-speed Tundra trans with it's 0.58:1 ratio is a bit tall for my preference (even with 4.30 gears) for a truck that drives hills, pulls loads, and lives at altitude, while running
only a 33" tire. Of course the tall top gears will tolerate very low diff gears.
I agree about the low gears, I love them on trucks and motos, particularly on modern vehicles with one or more overdrive top gear(s) that keeps the RPM
down and the MPG
up at moderate speeds. While newer engines are likely more tolerant of lower gears/higher RPM as stated, I would observe that the very tall (and/or multiple) overdrive top gears are a very real part of why newer truck platforms are tolerant of low diff gearing.
Even with low differential gearing, many trucks will turn very reasonable (even low) RPM at freeway speeds due to tall overdrives.