1) The torque curve for the pentastar is very flat. Hardly a curve at all. And it does come on pretty early.
Indeed. The problem is not the shape of the curve it is the anemic peak of the curve. Overall, this engine is an amazing accomplishment in my opinion. It gets a LOT done for a 3.6L normally aspirated engine.
2) When you go to a lower gear ratio in the Diff. you are doing more than making the engine Rev higher for any given road speed. You are also giving the engine an increased mechanical advantage at any given RPM. If I turn a hand wheel at 10 RPM and it is connected to another
Hand wheel of the same diameter via chain or belt I will perceive a certain amount of effort required. If I reduce the diameter of the driving wheel ( the one I am turning ) I can turn it at the same RPM as I was before reducing it and will not perceive an increased effort but in fact feel less effort. Albeit, the driven wheel will turn slower. That mechanical advantage made my work effort less. So gearing does in fact do more than up your RPM's in an effort to find a torque sweet spot. And we should remember the nature of the torque curve on this engine.
At the end of the day, the torque at the contact patch of the drive wheels is what is important. Changing the rear gear alone forces the truck to shift another gear up and the final ratio from the crankshaft to the tire is very similar. This is nice in first gear trying to get going up a steep hill, but the goal here is more focused on the hilly highway driving. The 8 speed transmission ensures that rear changes don't do a lot for the delivered torque.
The reason I am likely going to the do the gear change is (like you mentioned) to reduce the load on the transmission.
3) Had my truck on the open highway yesterday and decided to do a little experiment.
70 MPH in 7th gear = 2,000 RPM
70 MPH in 6th gear = 2,400 RPM
70 MPH in 5th gear = 3,000 RPM
That truck would run all day a 3,000 RPM. Point is, it would most likely do just fine at even higher RPM's in a lower gear when needed for a pull up a grade.
When fully loaded, the truck never sees 7th or 8th gear at all. We nicknamed 6th gear the 'Unicorn gear' because it is elusive - almost never makes it to 6th. When it does, not for long. 5th gear is really only for flat ground for the most part, cool temps, low-ish altitude, etc. The slightest grade pushes the truck into 4th, medium grades get into 3rd which gets pretty ridiculous if trying to maintain 65mph puts the RPM at 4700. Slowing down to 55mph gets me to 4000 rpm, which is still pretty serious AND I am going 25mph slower than all the other traffic which can get scary.
At the moment - the expectation is that I will go with a 3.92 rear-end which should help the transmission handle the additional 100+ ft/lbs of torque. I estimate that is should be able to maintain 65mph at 2700 rpm (6th gear) and be able to deal with grades up to around 5%. At 6% or so, probably going to 5th gear at 3500 rpm. Not too bad, not a lot of long stretches of 6%+ grades. Steeper grades, I would just slow down.
