I have a 2006 Chevy half ton with a 5.3 and automatic. the truck has 3.73 gears and 275/55/20 tires. the diameter of these tires is rughly 32 inches. At 32 inches with a 3.73 rear gear and what I believe is a 4L65E trans my tach says 3000 rpm at 80 mph....all of the online gear ratio calculators defy this. this truck was owned by a GM tech and has been "tuned" on his laptop for maximun performance. What gives?

 

Did he not recalibrate the speedo when he did the gear swap? It is very simple to do if you have tuning software.

 

Moved to a more appropriate section.

 

80 should be about 2200rpms..
Your speedo needs calibrating

 

3000 should be ~109