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Donnyjones
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2.85 gears with 18 speed

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Thinking of switching to 2.85 gear end ratios. I have a 2005 KW with C15 Cat and a 18 double overdrive transmission. I haul a hopper, so I'm always 80,000 pounds loaded. I would appreciate any feedback you can give me. Thanks.

Edited by: Kevin Rutherford ( )

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Rick Williams
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The reason we go to a gear ratio in the 2s is to be able to run in a direct drive gear, which gives us a 4% increase on fuel mileage. On your double overdrive 18spd, 18 and 17 are both overdrives. 16th gear would be direct on this transmission and that's the gear you would want to run in for best fuel economy. I'm not really up on the C15 Cat but I'm guessing you would want to run this engine in the 1250-1400 RPM range. I'm thinking with 2:85 rears the highest speed you would want to go would be about 58-59mph and 55mph or even a little less would be even better. You can always use 17 gear for faster speeds but if you do most of your running in 17th gear its defeating your purpose of the 2:85 rears. So I guess its a matter of how fast you want to do most of your running. I think you might want 2:79s and run at 55mph in 16th gear. This seems to be a really good combination. 18th gear on this setup will not be a usable gear, only for out running the cops. Hope this helps.

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Ronald Sanford
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I have 2.85 with an 18 speed with a Detroit series 60 I run in 17th 90% of the time at 63 mph it works great for me

I have X One XDA Energy

Ronald Sanford,

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Your
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What size tires on these rigs ?

Low Pro 22.5's

Donnyjones,

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Cliff Downing
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My recent truck order is coming in with 2.79 rears hooked up to an 18. Will run it similar to Ronald. I do a lot of heavy loads on two lanes with hills. I want to run in direct on a lot of that stuff. Then when on the regular 4 lane stuff, move up to 17th and cruise along. The truck, when it comes in, will be on wide based 22.5 rubber also.

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daddyj159
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I am weighing my options for switching to just a front locker diff with a tag as well and scrapping the tandem setup. I was thinking either 279's Or maybe even 290's. I just above saw that apparently 285's are an option as well. I just want to be clear on this.......Basically a 279 in direct is equal to a 370 with a double overdrive correct? Therefore a 290 should be roughly the same as a 390?? And I guess a 285 would be somewhere in the middle correct? I run 53 mph all the time. I have 370's right now on 22 low pro XDA's. If I do the single drive axle conversion I am thinking I could probably get away with a touch lower gearing in direct? Any info would be appreciated :)

The similarities you mention are correct. What you would choose would depend on speed/terrain/weight you normally run along with keeping the particular engine make/size/model in the sweet spot on RPM's. Use the road speed calculators like those from Eaton or Cummins Powerspec to determine which combination work best for you.

Cliff Downing,

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