tts

Paul I don't have mine hooked up so I can't remember
but here is the 411 on the VSS


The VSS (Speedometer) Calculator
Overview:
The H-D EFI system utilizes a Vehicle Speed Sensor (VSS) device to convey the vehicle's speed to the ECM. The VSS device is mounted on the transmission, and determines road speed by sensing the rate of rotation of a "tone wheel" or gear. This signal is sent to the ECM for processing, then conveyed by the diagnostic bus to the speedometer where it is displayed.



In the ECM, the signal must be "scaled" such that the ECM knows the correct road speed to use in its calculations and to send the correct value to the speedometer. This scaling factor is called the VSS PPU value, and is equal to the number of pulses received from the sensor every 1/25th KM of distance.



The scaling factor can be calculated if one knows the following information:

• The Transmission Sprocket tooth count
  
  
• The Rear Sprocket tooth count
  
  
• The rear tire "rollout" in inches (or cm)

Determining Tire Rollout:
The VSS Calculator utility provides a way to estimate tire rollout by entering a tire size and then calculate the VSS setting based on your vehicle's gearing.



Tire rollout is affected by tire construction, wear, inflation pressure, and loading. For this reason, the rollout calculator is only an estimate and may be off 1-2 percent. For the most accurate results, the tire rollout should be measured. This is easily done on any flat surface using the following procedure:

Stick a piece of tape to the floor alongside the rear tire

Using a marker, mark the tire sidewall and adjacent floor tape so the mark lines up between the two

Roll the vehicle in a straight line until the wheel has made exactly one full revolution

Measure the distance from the initial tape mark to where the vehicle now stands. This is your rollout distance



Using the VSS Calculator:
The VSS calculator is accessed from the main editing form under the "Tools" menu.




The VSS Calculator Form



First, select the units you want to work in: English or Metric

Next, select the vehicle year and model from the drop-down lists. The calculator supports 2001 through current Harley-Davidson vehicles. When a vehicle is selected, the OEM tire and gearing information will populate the form. If this information is not correct, edit it until it matches your vehicle.



Tip: If the Calculator was invoked from the Speedometer Calibration form during programming, pressing the Calculate VSS button will automatically transfer the calculated value to the New Setting edit box.



Calculating Rollout:
To calculate the rollout for a given tire size, select your tire dimensions from the three combo boxes and press Calculate Rollout. This will calculate the following information:

• Estimated Tire Diameter (inches or cm)
  
  
• Estimated Revolutions per Mile (or km)
  
  
• Estimated Rollout (inches or cm)

Additionally, the Rollout in the VSS section will be updated and the VSS calculation will automatically be run for this combination.



Notes:

The Rev/mi and Rollout values are corrected for tire loading, so will not correspond exactly to the diameter of the tire.

Harley uses some custom tire sizes with no exact "standard" equivalent. Rollout can be estimated using the following equivalent size settings:



Harley Tire Size

Equivalent Tire Size

MU85B16

145/85-16 78V

MT90B16

130/90-16 73H



Calculating VSS:
After rollout has been determined, simply enter the transmission and rear sprocket sizes into the form. Press the Calculate VSS button to calculate the VSS value for your vehicle. This value is entered during vehicle programming, refer to Speedometer Calibration for additional information.



The following chart is a guideline for the typical range of OEM VSS values:



Model

VSS Range

VRSC with 28/72 Sprockets

1328 - 1341

VRSC with 30/72 Sprockets

1200 - 1240

Touring

1950 - 2113

Dyna, Softail

1996 - 2241

Sportster 1200

2140 - 2354

Sportster 883

2300 - 2355



Note Regarding 2001 - 2003 Vehicles:
2001 through 2003 Harley bikes use a different method for displaying the speedometer reading than the early vehicles. For these models, the VSS signal is connected directly to the speedometer and the speedometer calibration is not adjustable by MasterTune. However, the speedometer then sends a modified version of the VSS signal to the ECM which processes this for use with internal speed-related functions. This internal ECM speed data can be calibrated with MasterTune. This will NOT affect the speedometer reading, but it WILL affect the scan tool (DataMaster) speed readout AND the odometer mileage calibration.



For additional information, refer to the section Speedometer Calibration.

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