Results
RPM at chosen speed
3,249 rpm
Speed at chosen rpm
64.6 mph
Formula / model
RPM = mph x rear gear x trans ratio x 336 / tire diameter, mph = rpm x tire diameter / (rear gear x trans ratio x 336)
Use the gear ratio and RPM calculator to see how tire size, rear gear, and transmission ratio change cruise behavior, shift points, and finish-line rpm.
Enter your current numbers or target values below, then use the live results to review rpm at chosen speed and speed at chosen rpm before you commit to the next parts or setup change.
Vehicle speed is the product of engine RPM divided by the combined multiplication of the transmission ratio, rear axle ratio, and a drive constant (336 for tire diameter in inches and speed in mph). Every gear in the drivetrain acts as a ratio multiplier between the crankshaft and the contact patch.
Changing the rear gear ratio from 2.73:1 to 4.10:1 increases engine RPM by 50.2% at identical vehicle speed. This directly improves acceleration by keeping the engine closer to its torque peak but reduces highway fuel economy due to higher cruising RPM.
The relationship between engine RPM and vehicle speed requires exactly 4 variables: rear gear ratio, transmission ratio, tire diameter (inches), and either RPM or speed as the known value. The formulas are:
The constant 336 accounts for the conversion from inches per revolution to miles per hour per minute. When the transmission is in direct drive (1:1 ratio), the engine RPM is multiplied only by the rear axle ratio before reaching the wheels.
The tire acts as the final gear in the drivetrain. A larger diameter tire covers more ground per revolution, effectively lowering the overall gear ratio. Increasing tire diameter from 26" to 28" — a 7.7% change — reduces engine RPM at any given speed by the same 7.7%. This is equivalent to swapping from a 3.73 rear gear to approximately a 3.44 rear gear.
Interactive — linked to form inputs above
The table below calculates RPM at 70 mph with a 27-inch tire and 1:1 (direct-drive) transmission ratio. Overdrive ratios (e.g., 0.70) reduce these values by an additional 30%.
| Rear Gear | RPM @ 70 mph | Character | Typical Use |
|---|---|---|---|
| 2.73:1 | 2,484 | Overdrive cruiser | Highway towing, fuel economy |
| 3.08:1 | 2,803 | Balanced street | Daily driver, mild performance |
| 3.55:1 | 3,231 | Street performance | Bolt-on muscle cars |
| 3.73:1 | 3,395 | Strong street | Cam'd / headers / intake builds |
| 4.10:1 | 3,731 | Drag / autocross | Purpose-built performance |
| 4.56:1 | 4,150 | Race-only | Short gears, high-stall converters |
A 1-inch change in tire diameter shifts the effective gear ratio by approximately 3.7%. Switching from a 26" tire to a 28" tire on a 3.73 rear gear produces the same highway RPM as running 3.44 gears with the original tire. Tire growth at speed (from centrifugal force) can add an additional 0.3–0.5" at highway speeds.
Modern automatic transmissions include overdrive ratios of 0.63–0.75 in top gear. A 4L80E with a 0.75 overdrive and 4.10 rear gears produces the same highway RPM as a 1:1 transmission with 3.08 gears — enabling aggressive axle ratios for acceleration without sacrificing cruise efficiency.
The torque converter stall speed determines the RPM where the engine begins driving the wheels. A higher stall converter (2,800–3,500 RPM) pairs with aggressive rear gears (4.10+) and large camshafts that produce minimal torque below 3,000 RPM. The converter does not affect cruise RPM calculations — only launch behavior.
These are the next calculator pages most likely to be useful once you have this result in hand.
Drivetrain
01Tire diameter and indicated-speed error between two tire sizes.
Drivetrain
02Converter slip from mph, trap rpm, tire diameter, and final drive ratios.
Performance
03Quarter-mile ET and trap speed from power-to-weight.
Why displacement determines the RPM range where the engine makes usable torque for gear selection.
How gearing and weight interact with displacement to determine real-world acceleration.
How bore-to-stroke ratio affects the RPM range where the engine makes peak power.
It estimates rpm at chosen speed and speed at chosen rpm from values such as rear gear ratio, tire diameter (in), and vehicle speed (mph).
Start with rear gear ratio, tire diameter (in), and vehicle speed (mph) because those are the core values that move rpm at chosen speed the most. Then refine the secondary inputs to match the exact combination.
It is a solid planning tool built around the stated formula and assumptions, but final results still depend on real measurements, hardware tolerances, tuning, and operating conditions.
Yes. Change the inputs to reflect your exact parts, operating target, or comparison scenario, then review how the outputs respond before you make the next decision.
A useful next step is to compare the result with Tire Size and Speedometer Error Calculator, Torque Converter Slip Percentage Calculator, and Quarter Mile ET & Trap Speed Calculator so the rest of the combination stays aligned.
