VSS & Speedometer calibration for different diameter tires

[v10tt]

If I'm putting bigger tires, and I want my speedometer to read correctly, where do I make the changes?

I see a "VSS Calibration" number in the speed sensor screen that reads "37".  This I beleive is a multiplier and increasing it would make the vehicle speed increase. I'm not sure this the value that must be changed in order for the speedometer to read accurate, or this numbers changes the speed on the AEM screen only.

Thanks!

[v10tt]

If I'm putting bigger tires, and I want my speedometer to read correctly, where do I make the changes?

I see a "VSS Calibration" number in the speed sensor screen that reads "37".  This I beleive is a multiplier and increasing it would make the vehicle speed increase. I'm not sure this the value that must be changed in order for the speedometer to read accurate, or this numbers changes the speed on the AEM screen only.
Car is a Viper, and the AEm controls the speedometer. It all works fine right now.

Thanks!

[SB]

For the Viper specifically, the EMS is controlling the factory speedometer, so the EMS vehicle speed will match the speed indicated on the speedometer. In most other vehicles, the speedo isn't controlled by the EMS.

The VSS Calibration is the number that you want to change. The easiest way to do it is to put the car on a dyno and adjust the VSS Calibration until the numbers in the EMS match the speed readout on the dyno.

[edit 11-25-2008: need to adjust TP1 settings also... additional information below]

[v10tt]

SB, thanks! I appreciate the answer.
Just for my own knowledge, is the VSS calibration linear,meaning the the current value will change the same percentage as the change in tire diameter or gear ratio, or you pretty much have to adjust the number until the speed from the dyno matches the AEm screen?

[SB]

I believe it is linear, but you should really check it using a dyno (or radar if you have easier access to that) to be sure.

[v10tt]

SB, I had a chance to drive the car the other day. I changed values in the VSS as I was driving, but I saw no movement in the speedometer gage. Stock value is "37" . I changed it all the way to "50", and nothing.

I have the V1.11 sofware, not the V1.19.

Is there anything you can suggest that I can do to trigger the VSS into changing?

I tried it the other day on another Viper with the V1.19, and it worked, at least it showed on the Laptop parameters screen..

Thanks in advance for your response/help!
JC

[SB]

For future reference, this has been resolved.

1. When you adjust the "VSS Cal" option (Setup>>Sensor>>Vehicle Speed Sensor>>Options), that will recalibrate the 'Vehicle Speed' parameter that is displayed in the AEMPro software, this is sometimes used by the EMS for boost control or traction control calculations.

2. The 1500U EMS uses the TP1 output on pin B27 to control the OEM speedometer. The TP1 output needs to be adjusted separately from the VSS calibration.
In the AEMPro software, open Setup>>Advanced Setup>>Tach/Speedo Control>>Options.
In the Tach/Speedo options window, adjust the "Frequency M" option, that will recalibrate the signal that is sent to the speedometer on the dashboard. If larger diameter tires are installed, you will need to decrease this number so that the speedometer reads correctly.


Thanks to v10tt for confirming that this is the correct way to adjust the speedometer output.

[v10tt]