The Formula
TREMEC publishes the classic road-speed relationship as RPM = MPH × gear ratio × 336 ÷ tire diameter. For a transmission-equipped truck, use the total gear ratio: transmission ratio × rear-axle ratio.
Example: 6L80 + 4.10 + 33-inch tire
At 65 mph in 6th gear: 65 × 0.67 × 4.10 × 336 ÷ 33 ≈ 1,818 rpm before slip. That is why a 4.10 rear gear can still cruise reasonably with a 6L80's 0.67 overdrive.
Tire Changes Effective Gearing
A taller tire covers more distance per revolution, so it makes the axle feel numerically “taller.” A shorter tire does the opposite. The useful comparison is:
New effective ratio = old axle ratio × old tire diameter ÷ new tire diameter.
Do Not Judge Rear Gear By Cruise RPM Alone
- First gear determines launch multiplication.
- The RPM drop after each shift determines whether the engine stays in its powerband.
- Vehicle weight and converter behavior change how much gear the truck can use effectively.
- Tire rollout can differ from the nominal sidewall size; measure the installed tire when precision matters.
Use the site's calculators: compare 3.73, 4.10 and 4.56 at the same tire size, then compare where every 6L80 shift lands. Gearing is a system decision, not a single cruise-RPM number.
Sources & Further Reading
These links are here so you can verify the numbers and go deeper into the original technical material.