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13 speed, 3:55 rears, 1600 rpms, direct drive transmision?

I am operating a 3406B cat, at 1600 rpm, 13th gear, 3:55 rears, on 11R 24.5 rubber, 59/60 mph. Is this a direct drive transmission. What is the mathematical formula?

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i turn 1600 rpm at around 70mph with a direct 10 speed on 445/50 r22.5 tires with 2.64 rears (translates to 3.55/8 with od). that means you probably have 4.10 or 3.90 with a single over - especially since it's that old of a truck. that's without doing the math though and just guessing. here's the formula: How to Calculate Your Rear End Ratio

Update

i did the math and you've got 3.9 rears with a single over. you're welcome ;)

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Slide up under your truck at the transmission, on the passenger side of the trans there should be a data tag on the main case with the model number.

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Sorry Jerry, they are 3:55, I replaced the front rear.

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so did you check the plate on the transmission? if you do have a direct drive 13 speed i'd like to make you an offer.

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I have just returned from a 2500 mile TX turn and spending much time doing the math, thanks for the link, it is a 13 sp direct drive. I am convinced that my tac is 250 - 500 rpm slow at 1600. I will contact my shop to see if it can be calibrated. My tire rpm is 473 I can forward my matrix from the trip. epmtrucks@aol.com

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I think you might be right about the direct drive, 1600 / 1 = 1600 / 3.55 = 450 / 480 (11R24.5) = .94 X 60 = 56 MPH

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assuming that a 11r24.5 tire has 480 rpm the ratio comes out to 3.33 at 60mph at 1600rpm. if he had a direct with 3.55 gears he wouldn't get up to 60mph :)

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I have a 13 double over and 3:58 gears, my RPM is 1350 at 60 MPH. I run LP 22.5 tires though. I think you might have a single overdrive, not a direct.

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how do you know if its double or single over? Does it have two Os in the RTO16713 like RTOO16713.

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