Well I installed an evolution and input the tire diameter. The edge now reads the correct speed on the edge pod. But my spedo in the cluster is still off. Will it correct the spedo on the cluster or just the edge unit?

 

Personally,I want my in-dash speedometer to read correctly. I don't care if this means that the computer is 4%-6% low. The only effect will be that the computer calculated MPG will be a little more accurate (it was always optimistic) and that the odometer will read a little low.

 

When the computer is correct, the speedo will read about 2 mph high.

 

At 60 MPH (according to a GPS), my CTS reads 59 MPH and my speedometer reads ~63. I plan to increase the tire size in the CTS by 5% to make the computer think it is going 56 and the speedometer read 60 at 60.

 

I THINK you want to decrease the TS value in your CTS to get the effect you want. That will "fool" the PCM into thinking you have gone a shorter distance for the same number of tire revolutions.

However, your dash speedo error 5% is much higher than normal, and you can't blame the CTS or even the PCM for this. The error is normally about 3%. I haven't looked at how the speedo is "driven", but it converts an electronic signal from the PCM to a mechanical readout (and is possibly a simple electromechanical voltmeter?). If so, you could probably wire in a small voltage reducing resistor in series in the speedo feed to correct it.

- Jack

 

I agree - decrease it is.

 

Recognizing that it is hard to differentiate 1 MPH on the dash, here is the differences: Dash --> CTS (%error)
20 --> 19 (5%)
30 --> 28 (7%)
40 --> 38 (5%)
50 --> 47 (6%)
60 --> 56 (7%)
70 --> 66 (6%)
80 --> 76 (5%)

My GPS reads 1 MPH or so higher than the CTS