Speedometer Anomaly - Brain Teaser
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,727
Likes: 20
From: Carson City, NV
Speedometer Anomaly - Brain Teaser
I discovered something wierd a while back. My '98 C5 M6 originally had 342 gears which I changed to 373. After trying out my Garmin GPS, I discovered something. While participating in the Nevada Open Road Challenge last May, I set my cruise control at 120 mph as indicated on my DIC and also matched my GPS, but the speedometer indicated 125 mph. Anyone have any idea what this means? Do I need to change the PCM program based on the gear ratio and tire size? I thought the speedometer got it's input from measuring the actual axle rpm which means the gear ratio should not matter. 
John
John
Last edited by Cscokd; Aug 25, 2007 at 01:28 PM.
Former Vendor
Joined: Apr 2007
Posts: 1,331
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From: Jacksonville Florida
I discovered something wierd a while back. My '98 C5 M6 originally had 342 gears which I changed to 373. After trying out my Garmin GPS, I discovered something. While participating in the Nevada Open Road Challenge last May, I set my cruise control at 120 mph as indicated on my DIC and also matched my GPS, but the speedometer indicated 125 mph. Anyone have any idea what this means? Do I need to change the PCM based on the gear ratio and tire size? I thought the speedometer got it's input from measuring the actual axle rpm which means the gear ratio should not matter.
John
John
Safety Car
Joined: Aug 2000
Posts: 4,933
Likes: 30
From: San Rafael CA
I discovered something wierd a while back. My '98 C5 M6 originally had 342 gears which I changed to 373. After trying out my Garmin GPS, I discovered something. While participating in the Nevada Open Road Challenge last May, I set my cruise control at 120 mph as indicated on my DIC and also matched my GPS, but the speedometer indicated 125 mph. Anyone have any idea what this means? Do I need to change the PCM based on the gear ratio and tire size? I thought the speedometer got it's input from measuring the actual axle rpm which means the gear ratio should not matter.
John
John
Thw IPC (the computer controlling the gauge cluster and the DIC) gets its data from the Vehicle Speed Sensor (VSS), as it shares the signal wire from the VSS to the PCM and EBCM.
So if the VSS says your speed is 120 MPH, then both the gauge and the DIC should display 120 .... the DIC is accurate because it is digital, the gauge is inaccurate as it is analog. A 5 MPH error at 120 MPH is about 4 percent ... not too bad ... at 35 MPH that same error rate would make the gauge read about 36 MPH.
