![]() In this case, with 38" tires and a 4.10:1 final drive, the SGR = 2.176030384 To calculate what gears you need, you first need to know the Speedometer Gear Ratio (SGR). That said, you first need to figure out what gears you have in your transfer case (you already know your tire OD and final drive ratio). The correct way for a speedo shop to calibrate a speedo is to ensure that the odometer reads correct at 1000 revolutions per mile, then make the speedometer head agree with that by either moving an adjuster in the speedometer head or by changing the magnetism of the flying magnets in the head. This is something I cut and pasted from a post of his on the 1st gen board. Then, just pick the driven gear that most closely matches that number.Īll I have to say is Steve Chin is THE MAN!!! You can goto this website and enter the numbers for the drive gear, rear gear ratio and tire measurement and it will calculate it for you. The DRIVEN gear is a cog-like gear that comes into the tailshaft section from the side, and the spedometer cable inserts into it.Īlso DO NOT go by the printed size on your tires, get a tape and measure the diameter from the ground to the top of the tire. It slides over it pretty much like a collar. The DRIVE gear slides over the rear output shaft (at least on my NP208). When you say the cae has a gear, are you meaning the drive gear or the driven gear? Case that is going into the truck has a yellow gear. Best guess the speedo is about 10 mph slower than actual speed in the 70mph neighborhood. The gears are the same whether they are in the transfer case (4x4) or in the tailshaft housing (2WD). Chances are good that you will be able to change the drive gear only and not the driven gear. Remember to use the true tire diameter and not it's rated diameter. That is a Ford speedo will read the same if swapped into a Chevy truck. This formula works for Chevy, Ford and Chrysler because speedometer cable speeds are the same for all of them. # teeth in Driven Gear = (Drive Gear Teeth# X Axle Ratio X (20168 / Tire Diameter))/1001 Just pull the speedo gear (driven) out and shine a flashlight in the hole. You'll have to find out what color your drive gear is. ![]()
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