best rpm 870

[cody]

Im putting a 870 in a 99 truck and am wondering what rpm they like for cruising and fuel mileage. I want to set it up to run 65 mph cruising speed and around 475-500 hp and 1850 torque. Once i know the rpm they like i can configure tire size and gear ratio to get end results.

[Rawze]

The optimum for longevity is the 1600-1800 range while cruising.

[3406E]

For reference I run 05 cm870 475@1850 9900 eagle . 3.73 gears 18913a. At 65mph my rpm is 1400 almost at 1500, was factory governed. I later took the governer off started crusing at 67 mph and rpm went up to 1500 1/2. At 70 mph the rpm is at 1600. I don’t really run it there unless I want to pass a truck or car quickly. When doing 70 through the rpm sounds really angry and loud so I always thought that was too much. Runnig at 67 I get good fuel milage especially on i10 and i20 because it’s flat. When grossed out if you don’t punch the rpm up to 1700+ when catching up to road speed youll be really slow. Truck is not deleted also

[tree98]

You don't have to use top gear. I never use 8th gear on my 8LL.

[mikkhh]

(05-03-2025 )3406E Wrote:  For reference I run 05 cm870 475@1850 9900 eagle . 3.73 gears 18913a. At 65mph my rpm is 1400 almost at 1500, was factory governed. I later took the governer off started crusing at 67 mph and rpm went up to 1500 1/2. At 70 mph the rpm is at 1600. I don’t really run it there unless I want to pass a truck or car quickly. When doing 70 through the rpm sounds really angry and loud so I always thought that was too much. Runnig at 67 I get good fuel milage especially on i10 and i20 because it’s flat. When grossed out if you don’t punch the rpm up to 1700+ when catching up to road speed youll be really slow. Truck is not deleted also

How much is the fuel consumption with gear lower revs 1600-1800 cruising at 55mph vs 65mph and 1400rpm.

55mph means less fuel consumption, less wear, more time to react to something unexpected happening

[3406E]

I never did 55 before, got ELD to worry about and truck stops getting full. Have to drive not in a rush but very productively.

[Rawze]

(05-03-2025 )3406E Wrote:  For reference I run 05 cm870 475@1850 9900 eagle . 3.73 gears 18913a. At 65mph my rpm is 1400 almost at 1500, was factory governed. I later took the governer off started crusing at 67 mph and rpm went up to 1500 1/2. At 70 mph the rpm is at 1600. I don’t really run it there unless I want to pass a truck or car quickly. When doing 70 through the rpm sounds really angry and loud so I always thought that was too much. Runnig at 67 I get good fuel milage especially on i10 and i20 because it’s flat. When grossed out if you don’t punch the rpm up to 1700+ when catching up to road speed youll be really slow. Truck is not deleted also

Sounds like its geared too tall to me vs. the speeds you want to run.

[Rawze]

(05-03-2025 )mikkhh Wrote:  ...
How much is the fuel consumption with gear lower revs 1600-1800 cruising at 55mph vs 65mph and 1400rpm.
...

rpm range vs fuel economy at the same exact speed is a mis-nomer unloess the truck is absoutely bobtail with zero load on it.. which is not the case on a truck that has to haul freight.

as soon as you put any kind of load.. even light load on the truck.. the rpm make absolutely ZERO difference unless your running 2,000+ or something unnecessary.

A fine example is my own truck. I run 55-58 mph everywhere and ... I easily get in the 8-9mpg range all day long with fully loaded 80,000 lbs, running 1750 rpm, a gear down. Matter of fact, it gets slightly lower mpg if I do go up a gear and lug it below 1500 or so when loaded.

This subject has been discussed on here before many times...

rpm range vs. gearing...
https://rawze.com/forums/showthread.php?...3#pid17353

An article on Hyper-miling (for extreme fuel saving nuts like me)...
https://rawze.com/forums/showthread.php?...8#pid24378

other related posts...
https://rawze.com/forums/showthread.php?...6#pid17356

[cody]

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.