Drivetrain
Solve for final drive ratio, engine RPM at any cruising speed, or the ideal axle ratio for your tire size and transmission. Used for re-gearing trucks, Jeeps, and pro-touring builds.
A gear ratio calculator solves engine RPM at a given road speed, or finds the axle ratio needed to hit a target cruise RPM. Final drive and tire height together determine whether your build lugs on the highway, screams at 70 mph, or tows a trailer in the power band.
Essential after tire upsizing, transmission swaps, or debating 3.31 vs 3.73 in a Mustang or F-150. Diesel owners use it to confirm overdrive pairing before a long haul.
A good outcome: RPM at speed you can compare to tach readings, or an axle ratio target to quote when ordering gears.
Tire diameter should be loaded overall height ÷ π, or use the tire size calculator — worn tires read smaller than new.
Automatics show converter slip at light throttle — RPM at cruise may read higher than this formula until lockup.
Verify in top gear (or the gear you entered) at steady speed with GPS and tach.
“Ideal axle ratio” mode needs a realistic target cruise RPM — many modern OD transmissions want roughly 1,800–2,200 RPM at 70 mph for economy.
RPM = (MPH × 336 × Trans gear × Axle ratio) ÷ Tire diameter (in)
336 is the conversion constant (mph, inches, minutes). Example: 70 MPH, 26.5″ tire, 0.70 OD gear, 3.73 axle → RPM = (70 × 336 × 0.70 × 3.73) ÷ 26.5 ≈ 2,078 RPM.
Rearrange for axle: Axle = (RPM × Tire) ÷ (MPH × 336 × Trans)
RPM = (MPH × 336 × Trans gear × Axle) ÷ Tire diameter (in).
Example: 70 MPH, 26.5″ tire, 0.70 OD, 3.73 axle → (70 × 336 × 0.70 × 3.73) ÷ 26.5 ≈ 2,078 RPM.
Rearrange for axle: Axle = (RPM × Tire) ÷ (MPH × 336 × Trans). Constant 336 ties mph, inches, and minutes.
A Ram 2500 with 3.42 rear axle, 0.67 sixth gear (Aisin transmission family), and roughly 33-inch tires at 70 mph often cruises near 1,600–1,750 rpm in the RPM-at-speed tab — typical for modern HD diesels tuned for towing fuel economy.
Jeep has long offered 4.10:1 axle ratios on Rubicon models (with NV241 transfer case and low-range for rock crawling). Off-road builds use this calculator to see how taller tires raise effective gearing after a lift and 35-inch swap.
Many owners target 2,000 rpm at 70 mph with a 0.70 overdrive and 26–27-inch-tall tires. The “ideal axle ratio” mode solves for rear gear — common when debating 3.31 versus 3.55 on a Coyote Mustang.
If calculated cruise RPM is off by more than a few hundred, check true tire diameter (not just aspect-ratio math on worn tires), whether you are in the correct transmission gear, and torque-converter slip on automatics (tach reads higher than road speed implies at light throttle).
Taller tires than spec drop RPM; regear swaps change both acceleration and highway cruise. Verify with GPS and tach at steady 70 mph in top gear before buying ring and pinion.
Taller tires lower RPM at the same MPH because the axle turns fewer times per mile. Many owners regear after 33″ or 35″ tires to restore cruise RPM and towing performance.
Many modern overdrives target roughly 1,800–2,200 RPM at 70 mph where the engine’s BSFC valley lives — but too tall a gear lugs the engine and can hurt MPG and towing.
Gearing does not change the truck’s GCWR sticker, but a lower (numerically higher) axle ratio improves launch and grade holding at the cost of higher RPM and fuel use on flat highways.
Pair this calculator with these related tools in the garage — same session, no signup.