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Re: Understanding your GenI Speedometer
10/10/04 04:09 PM
Great info Doug!!! :)
To add to this.. I just happened to be working on the same issue:
For 31" tires @ 60 mph the tire will turn 651 rpm.
A good piece of data is the factory speedo is calibrated for 1026 RPM @ 60 mph.
Example of the calulation to determine your speed gear for a 31" tire:
31" /12" =2.58 ft.
2.58 x 3.14 (pie) = 8.11 ft circumferance of a 31" tall tire.
5280 (feet in a mile)/ 8.11 (circumferance of a 31" tire)= 651 tire revolutions per mile.
Since 60 mph is a mile per min the tire turns 651 rpm.
With 4.62 gears the drive shaft RPM will be 4.62 x 651= 3008 rpm
The factory gear for my speedo has a 26 tooth speedo gear and a 8 tooth drive gear (main shaft).
That gives a:
26/8 speedo reduction (3.25 reduction ratio). so 3008 / 3.25 = 925 rpm at the speedometer (speedo is calibrated to 1026 rpm = 60 mph).
How far off is the speedo? 1026 / 925 = 1.109.
At an indicated 60 mph:
1.109 x 60 =66.6 mph! sounds evil to me....
Your odometer will be low the same factor.
So lets calculate what the speedo drive gear should be...
3008 rpm (actual drive shaft rpm at 60 mph) / 1026 rpm (what the speedo wants at 60 mph) = 2.93 (reduction ratio).
Since the 8 tooth shaft gear is fixed the only variable we have is to adjust the number of teeth on the driven gear: where x is the teeth needed on the driven gear.
X /8 = 2.93. We will never get an exact correct ratio since you the driven gear increments one tooth at a time.
The choices are:
So 24 / 8 = 3 to 1 reduction ratio
Or 23 / 8 = 2.875 to 1 reduction ratio
Yup a 24 tooth gear is as close as we are going to get.
Also the speedo has a reed switch to send the vehicle speed to the ECU. The switch puts out 4 pulses per speedo revolution. So at 60 mph the frequency would be 68.4 hz.
That style of speedo reduction gear looks like the same unit that was used on all the mopars I have ever worked on.
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