Looking for help/advice on truck purchase

[Rawze]

(08-13-2017 )RStewart Wrote:  I appreciate the breakdown of this. I never would've thought that a truck would get better fuel mileage running 70 vs 60 when heavy.

I think you may have seriously mis-read that... So I edited the post and clarified it a bit more. it was worded a bit cryptic.

(08-13-2017 )RStewart Wrote:  These particular trucks are geared all wrong. The 386s have 3.36s & the 579 has 3.25s. I guess I could run in 10th instead of using all the gears.

I could not agree more for anyone hauling up to 80,000 lbs. Taller gears only help those guys who don't haul any freight and/or are only having really light loads, as the extra hp output is wasted when geared with a better ratio. the idiots who came up with those theories about tall rear ratios and directs and all that other BS never got in a truck with 80,000 lbs in it and tried slinging it around on the roads all day. -- Text-book smart and road dumb!.

(08-13-2017 )RStewart Wrote:  ...
That's interesting that the sweet spot is so high.
...

As far as fuel efficiency goes, there is not real "sweet spot" for the ISX. It has a VG turbo and other things that make it very fuel efficient across the board unlike older engines of the past. However,.. when it comes to longevity vs engine load,.. 1600 will produce a lot less engine wear and increase lifespan while under medium/heavy engine output (fighting those extra forces because your climbing that hill, or maybe going fast and sucking more fuel like a fool).

When we grab a load and gross at 78 - 80k, we will sit on 1600-1700 rpm for hours and hours at one gear down + jumping in and out of neutral, letting it roll (we have a 10-speed) when in hilly country (PA and the most of northeast). Same thing whenever we get to any kind of need for hp to keep speeds up, like grabbing the higher rpm before slowing down as we are hitting a hill climb. We only grab that highest gear when its perfectly flat, and we are loaded to 80k.

On the extremely rare occasion where we would go 65+, we just left it a gear down and used the 1600+ rpm range when we were heavy. It is overall slightly more fuel efficient than the same speed in top gear (only when heavy though) than when only doing about 1400 rpm. It can also be seen on our pyro gauge with the probe mounted to the outside of the manifold. However, when we are loaded light or empty,.. the complete opposite becomes true. It all comes down to terrain, power requirements, and weights.

When we are light or empty, our entire driving style changes to minimum rpm instead of working it. -- When Heavy, the higher rpms are your friend and when empty, the lower rpms are your friend.

-- Any question on our driving style vs longevity and using a wide range of rpm to our advantage can be measured in the rod/main bearings in my engine re-build series on Youtube. As well, is the fact that our million mile fuel average to the pump by the receipts is 8.03 mpg and still slowly climbing.

My wife has been reporting last couple days or so the fuel receipts are showing 9.4 - 9.7 mpg, running 58~ish on the hazmat load from PA to new mexico. Load itself weighs only about 28k in a 53-ft dry box. Its nice to get those lighter loads once in a while. If i were geared 3.36's or 2.79's and direct,.. there would be no way in hells creation I could get those kind of numbers, simple as that.