cruising rpm?

[dansvan]

I'm running a 6.9 with a C6 behind it, and a US Gear OD behind that. (.75) with 4.10 gears. Tires are 9.5x16.5 which are about 29.5 inches tall. My Isspro tach says I'm running around 2500 to 2600 rpm at 65-68mph. According to the internet gear calculators I should be at lower rpms than this. Not much, but a little lower. So the questions are, 1. Is the converter loose on these trucks? 2. Is the Isspro tach "off?" 3. Is 2600-2700rpm going to hurt this motor over a long haul?

I have new 235/85r16 tires I'm installing as soon as snow flys, they are about 31.5 inches tall. This will lower the rpm's some. Just curious what rpm's you guys turn.

 

[Agnem]

Sounds right to me. I have a C6 with no OD and a 3.55. Your 4.10's cancel out your OD difference wise, so you should be about right. No lockup converter for the C6 unfortunately.

 

[6 Nebraska IDIs]

Whats the top speed before he would start loosing fuel milage?

 

[Diesel JD]

I'd say for the best fuel mileage you need to be in the 1400-2200RPM band and 2500-2600 you could still do pretty decent especially with a turbo. Most here will say you can run a 6.9 at governed RPM all day with no ill effects. Folks like Towcat and Midnight Rider have proven that it can be done. Is it good for the motor? Probably would be better on it if you were in that "sweet spot" 1400-2200, but not likely to damage anything unless something else is already wrong.

 

[Cheaper Jeeper]

Sounds right to me. I have a C6 with no OD and a 3.55. Your 4.10's cancel out your OD difference wise, so you should be about right. No lockup converter for the C6 unfortunately.

Ditto on my 2WD non turbo - also with a C6 and 3.55 gears, also around 2600 RPMs at 65.

On my 4WD turbo-ed truck with 4.10s and the ZF-5 OD (.77) I'm turning right at 2200 RPM at 65.

I would expect yours to be right in about the same range as my 4WD, dansvan, since we have the same gears and about the same OD ratio. I'm running the 235/85/16s already though, so your low profile tires may be the reason your RPMs are closer to the same as my 2WD....

The gearing difference between 4.10 and 3.55 is about 15% (4.10/3.55-1.15=115%). The difference between a 1:1 high gear and a .75 OD is 33% (1/.75=1.33=133%). The net difference of the combination is like having an 18% OD (133%-115%=18%).

The difference between the 29.5" diameter tires and the 31.5" tires is nearly 7% (31.5/29.5=1.1067=107%) That reduces your net gain from the OD to 11%. So your RPMs should still be about 11% lower than a 1:1 high gear & 3.55 diff gears combo. But yet yours are about equal. Have you checked your speedo against a GPS?

With the smaller tires the speedo should read a little faster than what you are actually going, but it almost sounds like it may be reading a little slower than your actual speed. Has the speedo drive gear been changed to compensate for the smaller tires?

 

[towcat]

7.3 turbo'd IC'd ZF5 4.10LS 650deg on the pyro.
lookie at the pics
others who have driven the truck will testify that it likes to cruise at this rpm/speed all day.

 

[Full Monte]

If you want mileage, you are limited by the C6 slippage and vehicle speed.
For a long time, I couldn't figure out how some guys were getting such good fuel mileage compared to mine. I think you have to do what guys like Mel do:
use a 3.55 rear end, a manual tranny, and an overdrive. Some use a 3.55 and a zf5 tranny, with an overdrive. If you do this, you might get down to 1600 rpm at 55-65 mph on the freeway, which seems to be the sweet spot. I haven't made it to that point, but would like to.

 

[Agnem]

While it is true that the IDI's don't seem to mind running at those high RPM's all day, it seems pretty appearant that on flat and level ground, above say 1800, the engine starts to consume more fuel to spin itself than it does moving the vehicle. I have a 5 MPG difference between the Moose truck (6800 pounds) and the Moosestang (5000 pounds) and a 1000 RPM cruising difference. The extra weight of the Moose Truck should by all accounts create a MPG difficiency comparatively, but the 1000 RPM's the Moosestang has to turn to keep the same pace results in 60,000 more revolutions, 120,000 more injections of fuel and an obviously higher fuel consumption per hour with that kind of differential. Low RPM's are key to good economy provided that stroke for stroke the engine is not working harder than it otherwise has to.