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.

In the dually I get 2000 RPM at 57mph on the cluster which is within 1mph of GPS. I have 225/75/16 tires, 454, 4L80, and 4.10 gears. I forget where I see 2500. In the 1500 with 285/70/17, 4.10, and 4L80 I see 2000 RPM at 62mph.

My guess is your gear ratio was altered at some point in its life. Based on the numbers given by each responder and myself in this spreadsheet. 62-63mph puts you at 4.56 gears.

Thanks guys. The 4.56 ratio did come into my mind as well. I think I will physically check the rear end to confirm.

I just switched gears 4.56 to 3.73. AT 65 mph I was arou d 2200 At 75mph I was around 2600. Now 65 is 1800 And 75 is 2150 My tires are 215/85/16 rear tires and 265/70/16 Front. Same height but wider