Posted by Charlie on Thursday, July 05, 2001 at 12:23PM :
In Reply to: 4BT rpms posted by mike (aka packrat) on Thursday, July 05, 2001 at 12:08PM :
That'd be just fine. Ideal gearing is gearing truck for governed speed (25-2600 with Cummins, 2800 with other motors) at 75-80 mph. This is too fast for sustained running due to vehiclular considerations like air resistance, noise and safety, so sustained running at 70 will be 170-300 rpm less. It doesn't hurt a diesel to run up to governed speed for a short while. for best economy, it should be run for long term at least 200, better 3-400 rpm below governed rpm. If you gear it too tall, it will lack power in top gear. That is only viable if you have a firebreather, like an ISB 16 valve 170 hp 420 ft-lb 4BTA, or a propane breather. I just added propane to my M37/Hercules, and I can now switch back from 36.9" tires to my 38.7" 11.00s, which will raise governed 5th gear speed from 76 back to 80. But I never drive sustained over 70.
If someone did a Powerstroke or a 6BT I'd say gear it for 85-100 mph, just for economy and quiet reasons.
Charlie
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