m05fastbackGT

Need help, as I recently changed my axle gear/ratio from 4:10 back to 3:55. According to Brenspeed, they said I can make parameter changes in their custom calibration tunes for the rear axle and tire revs per mile through the end user adjustment menu with the SCT X-4. After making the axle ratio change, I also changed the RPM by starting out at 775 RPM according to the specs from the Goodyear website for the Eagle F1 supercar tires which are 285/40/18. I then went ahead and did a speedometer vs GPS comparison test and found that my speedometer was reading 60MPH in 4th gear at 2000 RPM while the GPS was reading 40MPH. Needless to say, I knew from that moment my speedometer was way off by 20+ MPH when compared to the GPS. Afterward, I came across some tire/speedo calculation sites in which two of them claim the correct RPM specs for a 285/40/18" tire is 748 RPM. Their reason? their calculations are based upon the tires being installed on the vehicle while under load rather than uninstalled with no load according to manufacture specs. At any rate, I proceeded by changing the RPM setting from 775 down to 750 RPM on the SCT X-4 and did another test run comparison. According to the Speedometer, I was doing 60 MPH in 5th gear at 2000 RPM, but when compared to the GPS? the GPS was reading between 65-68 MPH while in the same gear and same RPM. Therefore following the second test run comparison, my speedometer is now off between 5-8 MPH slower when compared with my actual speed according to the GPS. I also made several other RPM adjustment changes ranging between 750-765 and then from 750 down to 720 RPM but despite the changes, my speedo was still off between the very same 5-8 MPH slower than the actual speed compared to the GPS which pretty much were the same results from the second test run with the 750 RPM selected. Anyhow my concerns are this, after speaking to someone from SCT tech support, they claim that even if a custom tuning vendor provides the end user to make parameter adjustments to any of their custom tunes, the adjustments will NOT change the parameter settings and claim that the vehicle's ECM will still have the 4:10 gear ratio from the original custom tune files and would, therefore, require a new tune revision from the tuning vendor that includes the new 3:55 gear ratio change, otherwise any adjustment change from the end user will not download from the device into the vehicle's ECM. That being said, would this possibly explain the speedometer issues I've been having? In the meantime, If anyone can offer any feedback/input? I'd really appreciate it.


Thanks.