What's the actual height of your tires? I've always been a big fan of this site... http://www.4lo.com/calc/gearratio.php
Builds Cummins 6bt/nv4500 build (2 Viewers)
- Thread starter Mr Cimarron
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- #542
When I entered 315/75 into the page it showed 34.7" or something like that.
I was thinking that myself. What if the tach is not dead on??? Does it feel under or over optimal Rpms at 70?
Have you verified your speedometer with GPS also?
Do you have yours reading off the flywheel? I think you have to input the number of teeth in a full rotation or something like that.
Dirty Koala
SILVER Star
Cim has a dakota digital reading off the alternator I believe.
I wonder if the size of the pulley affects it. I had to increase my pulley size to get better belt wrap, so maybe they did the same during the install.
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- #548
Speedo has been checked with GPS. It feels like it's running good at 70 so my guess it's a tad off.
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- #549
That could be a possibility. I believe they went through a few before the right combo was found. I need to dig out the Dakota Digital and read the instructions.
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- #550
Correct.
So here is my understanding of what the Dakota Digital tach converter is doing and why the RPMs are off. I have this in my truck but it runs off of a sensor that counts the teeth on the flywheel so this is specif to that setup. It should be applicable to the alternator driven one though too, I am just not sure how to fix that setup.
The converter is really designed for V8 applications. That means that it it is counting the number of teeth per one revolution, in my case its around 140 if I remember correctly. It is then sending 8 pulses to the Tachometer to generate your RPM reading. The problem is that the LC is looking for 6 pulses per revolution not 8. This results in the RPS reading higher than they actually are by 33%. To get your true RPMs multiple the reading on the tach by .75. I fixed mine by multiplying the number of teeth by 1.33, that results in 6 pulses going to the tachometer every revolution rather than 8 pulses.
I am not actually sure if that's what is really going on but that is my theory and it fixed my tach reading.
As an example, my tach was reading about 2500 cruising at 70mph. If I apply the correction (0.75) that results in 1875 RPMs, a much more reasonable number.
What are your axle gears? Have you added T/c gears?? So what your saying is the 6bt nv4500 split case is perfectly happy at 75 at about 2200 rpms????
Stock axle gears, I have the Marks +3% OD in the TC I think (cant remember the ratio, same as Cimarron), 37" tires.
My truck has a 4BT, not a 6, just for reference.
I cruise at 70 MPH (per GPS) at about 1900 RPM and it hums along all day and get about 20MPG.
My calculations show that I should be at about 2000 RPM at 75 MPH.
My truck has a 4BT, not a 6, just for reference.
I cruise at 70 MPH (per GPS) at about 1900 RPM and it hums along all day and get about 20MPG.
My calculations show that I should be at about 2000 RPM at 75 MPH.
Sounds about right. I get the same except I'm at 18mpg because of all the weight and non aerodynamic rtt and roofrack.
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- #556
Mark's high gear is .92:1 so 8% reduction. Other than your 37s yours is the exact combo so I may fiddle with the DD and see if I can't match your RPMs.
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- #557
I think I will ballpark my tach to read this. 5th gear going 70.
I'd physically measure the tire of I were you. My 35's (315-70 17) are actually right at 34in diameter.
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- #559
I took the 34.8 off Goodyears website. I won't be able to get it exact but within 5% at 1900 is +/- 95 so thats good enough for me. Right now I seem to be off around 15%
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- #560
I guess I could buy a cheap RPM reader thing but really don't want to.
Anyone have luck with them?
Anyone have luck with them?
