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I am in the process of replacing the OEM 6.2 / TH400 / NP208 drivetrain in my M1028 with the P400 / 765R4 / NP241 I have been assembling for the past few years. Over the weekend I got the new powertrain bolted in place and decided to pre-determine the "driven" gear I would need in order for my speedometer to read correctly with the new 765R4 transmission. For those who may not know, the speedometer is controlled by a "Drive Gear" inside the transfer case (regardless of which type) and a "Driven Gear" mounted inside a removable sleeve on the outside of the transfer case, to which the speedometer cable is attached. GM offered (and still offers) various teeth counts on both "Drive Gears" and Driven Gears" so that the speedometer is correct to the ground speed, based on different combinations of "Drive" and "Driven" gears. The "Driven Gear" is easily changed from outside the case, however the "Drive Gear " can only be changed with the transfer case disassembled. Based on Item #12, Figure 83 in the 34P TM, the Drive Gear in a NP208 is GM part #14037997 which is a gray 15-tooth Drive Gear.
So here is the formula to determine the correct Driven Gear to use based on axle ratio and tire size:
# of Drive Gear teeth X axle ratio X (20168 divided by tire diameter) divided by 1001 = # of Driven Gear teeth to use
This can be further studied at http://www.73-87.com/7387garage/drivetrain/speedo.htm
To properly use the formula, you must first determine your actual diameter of the tires on your truck (assuming all are the same size, type, and mileage). To do this, you can easily jack up one wheel and use a tape measure to obtain the circumference in the middle of the tire, then by using the formula D = circumference divided by 3.1416. This will provide the Diameter value to use in the formula.
I am also attaching a PDF with additional information. Please note that there are two different outer sleeves used for different ranges of tooth counts.
Hope this helps. Administrator - you may want to consider adding this to the Stickeys.
So here is the formula to determine the correct Driven Gear to use based on axle ratio and tire size:
# of Drive Gear teeth X axle ratio X (20168 divided by tire diameter) divided by 1001 = # of Driven Gear teeth to use
This can be further studied at http://www.73-87.com/7387garage/drivetrain/speedo.htm
To properly use the formula, you must first determine your actual diameter of the tires on your truck (assuming all are the same size, type, and mileage). To do this, you can easily jack up one wheel and use a tape measure to obtain the circumference in the middle of the tire, then by using the formula D = circumference divided by 3.1416. This will provide the Diameter value to use in the formula.
I am also attaching a PDF with additional information. Please note that there are two different outer sleeves used for different ranges of tooth counts.
Hope this helps. Administrator - you may want to consider adding this to the Stickeys.