Start With The Published Ratios
GM publishes the 6L80 as a six-speed automatic with a very deep first gear and two overdrives. These ratios are the correct starting point for road-speed math; do not guess from memory or copy values from a different transmission.
| Gear | Ratio |
|---|---|
| 1st | 4.02:1 |
| 2nd | 2.36:1 |
| 3rd | 1.53:1 |
| 4th | 1.15:1 |
| 5th | 0.85:1* |
| 6th | 0.67:1 |
| Reverse | 3.06:1 |
Road Speed Is Math — The Completed Shift Is Not
The basic theoretical RPM relationship is RPM = MPH × transmission ratio × axle ratio × 336 ÷ tire diameter. It assumes no converter slip and an accurate tire diameter. That makes it excellent for checking whether your commanded shift RPM and commanded shift MPH are internally consistent.
A 33-inch tire, 4.10 rear gear and 0.67 sixth gear calculates to about 1,818 rpm at 65 mph before converter/tire slip. If your log is nowhere near that with the TCC locked, verify tire calibration, gear ratio, actual transmission gear and slip.
Why A Shift Happens After The Command
A modern clutch-to-clutch transmission has to release one element and apply another. Sonnax's hydraulic explanation of the 6L80 shows that the TCM controls clutch apply pressure through variable-force solenoids and regulator circuits. That means the shift event has a duration: the command, torque intervention, clutch fill/apply and RPM drop are separate moments.
What to log
- Engine RPM and vehicle speed.
- Commanded gear and actual/selected gear when available.
- Transmission input and output speed when supported.
- TCC commanded state and TCC slip speed.
- Throttle position / accelerator pedal and delivered/commanded torque channels when available.
- Transmission fluid temperature during repeat testing.
For a side-by-side view of 4L60/4L80/6L80 ratios, see GM Truck Transmission Ratio Reference.
Sources & Further Reading
These links are here so you can verify the numbers and go deeper into the original technical material.