Good morning. I am curious what RPMS you guys are seeing with vortec 454, and automatic with OD in crew cab dually. Based on GM code, I have 4.10 gears. I have original tire size. My speedo is definitely off. At 2500 RPM in OD it says 70mph but GPS says 63. The 4 variables are gear ratio, tire size, RPM and mph. I have done the math and something does not equate to what I am seeing. I have taken into account the .75:1 ratio of OD in trans in my math but still off. Thanks.