I calculated that for a 3.9 axle with a .69 overdrive ratio with 235/75/r15
tires the engine will turn 2030 rpm. With a 3.55 axle ratio the engine will
turn 1800 rpm. My truck has the 3.9 axle ratio with an o/d and this is the
rpm it turns. It seems like Bryan Harper's truck either has a 3.55 axle,
bad tach, bad speedo, wrong speedo gear, or bigger tires (31").
Jeff Painter, you said you are ordering a 2wd, I think these come with
205/75/R15 tires. If that is true than with a 3.90 axle @ 65 mph the
engine will turn 2170rpm, with a 3.55 axle it will turn 1970rpm. My
recommendation, if your going tow or have a lead foot for take off (like
most of us on the list) get the 3.90 axle, if you want good mileage get the
3.55 axle.
Brent Cring
NVG engineer
91 V-8 4x4
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