best rpm 870

[Rawze]

(05-03-2025 )cody Wrote:  Rawze you say you get better mileage when you run one gear down do you suppose that has something to do with that your are running in a direct gear instead of turning an overdrive which is harder to turn. I know that some of the bigger companies have gone to direct drive transmissions and have had a couple trucks that came with the 16 speed ultra shifts direct and even tho we turned the same rpms my other ones had 13 speeds and the 16 speeds would out pull them in the rolling hills when we stayed in the top gear. im always curious about messing around with gear ratios and tire sizes but to cheap to really go out and give it a valiant effort.

It don't matter what some idiot bean counter thinks or has calculated on paper as theory with regard to a truck once its heavily loaded. The final set of gearing (gear ratio of the rear ends themselves) is what makes or breaks it for a heavily loaded truck. Test drive enough different trucks when heavy, and you can clearly see that with taller rear ends, the more sluggish, lagged, and therefore less fuel efficient the whole thing is.

This is because the more torque load you put the drive-line thru (taller rears, direct drive, and higher torque loads on all the components), the more absorbed energy that the suspension, bushings, mounts, twist, and everything else in the whole damn truck loads up with + tranny heating up more, and everything else not accounted for absorbs too much energy vs simply gearing the damn rear ends right and getting things spinning faster (and therefore a lot less torque load on everything) up to that final rear end gearing where it all gets transferred much more efficiently into the tires.

Higher torque and less rpm (to produce the same power) also contributes to a LOT more wear on all those components too.. especially the engine (low rpm and high torque significantly shortens the lifespan of an engine), wears out all the gearing in the tranny faster, wears out the clutch faster, wears out the u-joints faster, mounts., etc.etc.etc.

Matter of fact, a set of 3.90 rears waay out-performs a set of 2.73's and direct drive in every scenario (same driven speeds, driving it at the same acceleration rate for test purpose, etc.) once the truck is heavily loaded.

I.E.> Any over-1:1 theoretical ratio loss is waay less significant vs. all the damn compression of bushings, twisting and stressing of all the joints, axle suspension, frame, + heat generation in the tranny, etc. that are all stealing away power due to the higher driveline torque itself. - Create the power for the wheels themselves by spinning every damn thing in the driveline faster, even if its above 1:1 ratio, .. and let that final gearing in the rears do all the hardest work for transferring it all in one shot where, where there is no bushings, springs, suspension, truck-frame, joints, or anything else that is going to twist and get loaded up.


That is what those idiot bean counters don't account for with all their 'theory'. The reality vs. some clown with a calculator is quite different once applied.


So.. my statements have noting to do whatsoever with direct drive. Matter of fact, every single truck that I have driven or test driven with direct drive gearing also had to have waaay too tall of rear ends for an actual loaded truck. It absolutely sucks to drive one once it was loaded heavy, and especially up any hill. Tall rears take measurable power away from the road/tires for the reasons stated above ... and was it was certainly NOT more efficient. Your barking up the wrong tree with that one.

And yes .. the mega-fleets have all gone to excessively tall rear ends to save fuel at the direct expense of the engine life, so that the engine cannot ever make full power, and is lugged to an agonizing death instead, in attempts to control their drivers right foot that stays planted in the floor all day with no regard to fuel economy. - There are articles about just that on the forum here.. here are a couple of them...
ref: https://rawze.com/forums/showthread.php?...6#pid32066
ref: https://rawze.com/forums/showthread.php?...4#pid71474

Its a trade-off. Increase fuel mileage across the fleet to combat the drivers holding the pedal against the governor all day by using taller gearing in the truck that is so exceedingly tall, so that it the engine is unable to make full power., Less power out = saving fuel, riding the engine against the bottom of its power curve, torturing it with high torque/low rpm and lugging badly, cutting its lifespan in half or less.

Engine lifespan don't matter to them, as they are going to sell the thing before the warranty is out any ways... and the next poor soul who buy that used mega-fleet truck gets an engine that has the equilivent of 1mil+ miles on it of wear, and is now in need of an inframe because truck rears were geared way too tall.

Its damn near impossible these days to find a used truck on the marked that has not been spec;d that way and not been abused to death before you but it. So yeah ... you can thank the bean counters at the mega-fleets for coming up with that horrible idea that abuses the equipment for fuel gains and puts all the problems that result onto the next owner.

[cody]

rawze i follow what your saying on the high speed rearends. Now back to the first part of my question do you think that when you are a gear down and getting better mileage that the 1 to 1 ratio is helping just as much as the higher rpms or is it not even a factor ?