I think you are wise in taking a slow and deliberate approach to the gear change issue. I have a big old Cat in my motor home, and I have found that it definitely has a sweet spot around 1400 RPM. If I go lower or higher, I lose FE. The sweet spot corresponds to the minimum engine speed to keep the rig in top gear. Don't do anything until you can find a torque curve.
Quote:
Originally Posted by t vago
I have considered a taller gear ratio, as well as a Laycock-type overdrive unit. I could go down to a 3.23 rear end with a taller gear, or I could effectively go to a 2.77 rear end with the Laycock unit.
Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck. Either option would push my engine speed well below 2000 RPM at cruise (1800 with the 3.23 diff, and 1560 with the Laycock unit), and any gain from running at lower engine speeds may likely be offset by the fact that the engine would be running well into the bad part of the torque curve. Nobody's been really able to produce a dyno chart for my engine (apart from the OEM propaganda), showing torque at speeds below 2000 RPM, so I can't really tell for certain.
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