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Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by RingMaster4x4, Jul 8, 2003.
Does any one have a part number or a source to get one from? I have a NP 205 with 4.56 gears.
if you know how many teeth you need just go to the dealership and tell them. they are exspensive though. something like 50 dollars apiece.
How can I figure out how many teeth need to bo in there?
this is what the good Dr. Steve Chin described it to me. ill do some more looking and see if i can find something else. theres another formula i just have to find it.
<font color="blue"> "Speedometers and odometers are calibrated for 10,000 revolutions in a mile. You need to calibrate the gears in the transfer case to obtain a gear ratio of 2.35:1 to get the correct odometer reading, then calibrate the speedometer head to jive with that. Most likely, you will not be able to achieve the exact gear ratio and you may need to go with something that's somewhat out of whack and use a drive ratio adapter (I have a 20.2% overdrive adapter on mine that was installed by the factory because the factory set my TC up for 3.73s and 30" tires).
FWIW, this is the formula for speedometer gear ratio: (63360 * Axle Ratio) / (3141.6 * Tire OD in inches) = SGR.
Take the SGR and multiply it by the number of teeth on the drive gear to obtain how many teeth you need to have on the driven (output) gear: SGR * DGT = OGT " </font>
the long explanation the same as steves i think. there is another way at the bottom
In most vehicles, the speedometer calibration is based on a certain tire and transmission ratio. These factors were used by the speedometer manufacturer to establish the proper combination of gears and worms inside the speedometer head to obtain accurate speed readings. Some car manufacturers and individuals building customized cars are using different tires and/or transmission ratios rather than OEM equipped. If this is the case, the ratio has to be established. In general, if the tires or transmission, including differential of your car has been or will be altered by you, or if any other changes have been made that are not listed as original or factory approved equipment, the ratio has to be adjusted. The following tests will tell you if the error you are experiencing is in the speedometer gauge itself or is caused by cable delivery from the transmission.
THE EASY WAY TO TEST RATIO
Stop at a mile marker found on most US highways. Record the odometer reading, including the 10ths.
Drive 10 miles and stop at the mile marker. Record the odometer reading again, including the 10ths.
Subtract the first reading from the last reading. This will tell us if the cable delivery (ratio) is correct or incorrect. If correct, your subtraction answer will be 10.0.
Conclusion: If odometer reads correctly per 10 miles driven, the error you are experiencing is in the speedometer head. If reading is incorrect, the error is from your transmission and the ratio must be corrected. Although rare, you could have both cable delivery and speedo head problems. In any event your cable delivery must be correct in order for the speedometer to be accurate. Contact APT.
FOR AMERICAN MADE CARS Generally, American made vehicles with mechanical speedometers need 1000 cable revolutions per mile to make the odometer record correctly and the MPH to be accurate. Here's a simple formula for you to fix many American made car ratio problems. Conduct the 10 mile test, remove driven gear (the gear that spins your cable), count the teeth, and then multiply the teeth times the subtraction answer from the test.
Example: 8.2 reading per ten miles actual multiplied by 38 teeth on driven gear (8.2 X 38) = 311.6, which means you need a 31 tooth gear to correct the error. In other words, 8.2 miles per 10 miles actual is 820 cable revolutions and you need 1000 revolutions to be correct. If you find that you cannot buy a driven gear with the right number of teeth, contact APT who can build a ratio adapter to fix the problem.
a calculator here but doesnt help with the teeth question
<font color="green"> here is a simple one go the link for the chart
Gear changes, transmission swaps and tire sizes. All these changes can cause problems with accurate speedometer readings. These charts should help you narrow down the selection of the proper gear combination to use in the transmission and speedo cable to 'get you legal' again.
Here is how this works.
You need to know your rear end gear ratio.
You need to measure the the diameter of your tire.
Formula is: gear ratio X 20.2 (another magic number) and dividing by the tire diameter.
Look up the resulting number in the chart and see what the closest match is. Use those gear combinations and you should be 'up to speed'!
Example: 3.55 X 20.2 =71.71 divided by tire diameter 26" = 2.758
After looking this up on the chart (large gears) we see that a 16 tooth tranny gear and a 44 tooth speedo gear will give us an accurate speedometer reading for this combination.
Yes, there is more than one 'correct' answer.
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