Rear-End Gear Ratios

[Wiseman]

(08-09-2017 )ZeroNthedark Wrote:  This is a question I've been wanting to ask for a long time, but not known how to bring it up...

My daily driver is a 2005 Pete 379 tri-axle dump truck(so a brick, basically). It has 425 steers and 11R-24.5 drives with 4:10 gears pushed by a 475 CM 870 and a manual 18-spd. Now I realize Hammerhead has had work done to his, and mine is in serious need of help, so we'll discount that for a minute, but why does it seem that if I drive 70-ish, at around 1600 RPM all day, my mileage appears better than if I go around 55-62, which is either 1400 in top gear or 1550 down a half gear? I usually try to keep my Boost under 30, mostly around 20, and Pyro hovers around 800 on tall hills, sometimes 850. This is in Western PA, so it's pretty much you're going up a hill or coming down. Very little flat ground on my runs except for I-76 between the 13 and 75. Which I don't get very often.

Sorry if I got long-winded or hijacked the thread, just looked like a good time to ask.

BTW- Best mileage I've managed is low 6's. Usually around 5.5.

Driving a dump truck in western PA have little to almost nothing to do with driving a truck on a fairly flat roads west of PA.
So , what Rawze and others here said about slowdown and hear the "chaching" sound is total truth. In the same time , going constantly up and down hill with loaded dump,the higher speed (inertia/momentum)your truck have the easy will roll uphill. Less shifting and less losses from turbo trying to spool again and build boost and so one . And let's don't forget. CM870 is different animal with air actuated VGT and responds way slower than electronic one on CM871 and later models.
Given all this things is nonsense to do any comparison.
On other hand , if your truck get touched by Gearhead, Unilevers or Mr Hagg you'll gonna see better mpg numbers for sure .