Gear Ratio Change? How tall can I go?

I understand that. The other question is what your 6th gear ratio is. That calculator by default puts a 1.00 ratio in the tranmission gear entry. That is usually your 5th gear on a 6 speed tranmission. Your 6th gear should around a .70, maybe a little higher or lower.

 

Unless that truck is running a direct drive tranny, some of those box truck are spec'ed pretty inexpensively. A lot of them run a DD tranny and do NOT run a tranny cooler, to save money when ordered.

As far as your question goes, I really don't think that you are going to want to dip much below 2,000 rpm, I know it sounds like a lot but that is a small motor and doesn't have a huge tq curve.

You start spinning that thing to slow and at only 215hp it will become a real slug. JMO

 

I know this old, what was the final decision for the original poster