RPM Calculator
Calculator
Enter your ring & pinion gear ratio, tire height, and transmission to see your engine RPM across highway speeds. Perfect for dialing in your Duramax build after a gear swap or tire size change.
Calculate Your RPM
Enter your ring & pinion gear ratio, tire height (in inches), and select your transmission — then hit Solve to see your RPM curve from 50 to 100 mph.
DMAXSTORE makes no guarantee as to the accuracy of these calculations. Results assume top gear at the listed final drive ratio. Contact us with any questions or corrections.
GM Duramax Rear Axle Gear Ratios: What Came in Your Truck?
Understanding your factory rear axle ratio matters when tuning, lifting, or swapping gears. GM kept it straightforward across the Duramax lineup — here's what you need to know.
2500/3500HD Duramax 2001–2025
From the factory, every 2500/3500HD paired with a Duramax came with one of two rear axle ratios. Trucks built from 2001 through 2019, covering the LB7, LLY, LBZ, LMM, LML, and early L5P, all used a 3.73:1 rear axle. This ratio worked hand in hand with the Allison 5 speed and later 6 speed to deliver the strong towing performance the Duramax is known for.
When GM introduced the 10 speed Allison in 2020, they moved to a 3.42:1 rear axle. That ratio applies to every 2020 and newer 2500/3500HD Duramax regardless of cab, bed, or trim. The numerically lower ratio isn't a step backward. The closer spaced gears of the 10 speed more than compensate, keeping the engine in its torque band when it matters most.
1500 Duramax 2019–2025
The 3.0L Duramax in the half ton platform is a different story. All 1500 Duramax trucks use the 10 speed automatic, but the rear axle ratio varies based on how the truck was optioned. The most common configuration is 3.23:1, which comes standard on most trims.
Trucks equipped with the Z71 or Diesel Off Road Package step up to a 3.73:1 rear axle, which also brings a larger 9.5" ring gear. The same 3.73 comes with the NHT Max Trailering Package, where the 1500 Duramax unlocks its highest tow ratings. If your truck left the lot without one of those packages, you're almost certainly on 3.23s.