'05 GTO Speedo Issues

OK, so I've had nothing but problems trying to get my speedometer to read correctly. It's been going on for about..well almost 8,000 miles according to my horribly inaccurate odometer! haha

Details
2005 GTO 6-Speed
3.89 rear gear DSS 9-inch conversion.
*built trans at the same time as rear*

I've changed the gear value, nothing. The tires, nothing. Then I tried an SCT handheld that simply asks you what gears you have so it's fool-proof, NOTHING!

HP Tuners will read and write correctly and the changes are going in, but the speedometer and odometer are showing the incorrect values. Approximately 10mph off.

I'm being told I have to recalibrate my DIC, but I don't know how or where to do that. Could anyone shed some light on what I could possibly be doing wrong with this car?

Thanks,
Kevin

I am running into a similar issue but with 4.11s. I am playing with tire sizes now and verifying via tomtom. It changes it more than me just telling it what gears were in it. I was ten to 12 mph off at first, flashed in 4.11 value and I was down to 7 off. Changed the tire height and the Revs per mile setting and i got down to 4 mph off.

why dont you upload your file here

and give me your Exactly Tire number with your Axle number

let me see it please

I had the same issues when I changed my CTS-V from the stock 3.73 to a 3.42 rear diff. It would display 71 mph at 2000 rpm, but my GPS was showing an even 80. My mileage was also coming up short as it was not counting correctly. You're right about the gear ratio box doing nothing; you'll have to modify your VSS output pulses per mile.

To do this, devide the displayed mph by the actual mph (i.e. 71 / 80 = 0.8875), then multiply your VSS output pulses by this number. Insert that number as it will be the actual pulses required for your speedo. After doing this to my -V, my speed now displays within .1-.2 mph of the GPS.

Give it a try and let me know how it works.