Results
Theoretical rpm
6,101 rpm
Converter slip
11.46 %
Formula / model
Theoretical rpm = mph x rear gear x high gear ratio x 336 / tire diameter, slip % = (actual rpm - theoretical rpm) / theoretical rpm x 100
Use the torque converter slip calculator to compare actual trap rpm against theoretical rpm and see how hard the converter is slipping on the top end.
Enter your current numbers or target values below, then use the live results to review theoretical rpm and converter slip before you commit to the next parts or setup change.
Torque converter slip is the difference between engine RPM and the RPM that would be predicted by vehicle speed, tire diameter, and gearing alone. The converter multiplies torque at launch but does so by allowing the impeller (engine side) to spin faster than the turbine (transmission side). Slip percentage quantifies this speed difference.
This calculator compares the actual trap RPM (from a datalogger or tachometer) against the theoretical RPM (calculated from speed, gearing, and tire size) to determine slip percentage. Typical values are 5–10% at cruise and 15–30% under hard acceleration in a non-lockup situation.
At 124 mph with 4.10 gears, a 1.00 high gear, and 28" tires, theoretical RPM is 6,108. If the tachometer reads 6,800 RPM, slip is (6,800 − 6,108) ÷ 6,108 = 11.3%. This tells you the converter is still multiplying torque at trap speed — common with loose, high-stall converters.
With the lockup clutch disengaged, 3–8% slip at steady cruise is normal for street converters. Under wide-open throttle acceleration, slip rises to 10–25% depending on stall speed. A converter that shows 0% slip at WOT is either locked up or the stall speed is far below the engine's power band. Excessive slip (>30%) suggests the converter is too loose for the combination, wasting power as heat.
Interactive — linked to form inputs above
| Converter Type | Stall Speed | Cruise Slip | WOT Slip |
|---|---|---|---|
| Stock lockup | 1,800–2,200 | 0% (locked) | 3–8% |
| Mild performance | 2,400–2,800 | 3–6% | 8–15% |
| Street/strip | 3,000–3,500 | 5–10% | 12–22% |
| Full race | 4,000–5,500 | 8–15% | 18–30% |
The stall speed determines the RPM at which the engine can hold against the brakes. A converter that stalls at 3,200 RPM lets the engine launch in its power band if peak torque starts at 3,500 RPM. A converter stalling at 1,800 RPM forces the engine to accelerate through its weakest RPM range off the line.
All converter slip becomes heat in the transmission fluid. A converter operating at 20% slip under WOT acceleration can dump 50–100+ kW of heat into the fluid. Street/strip converters with high slip need auxiliary fluid coolers and high-temp synthetic fluid to prevent thermal breakdown and clutch glazing.
A non-lockup converter wastes 3–8% of engine output as heat at cruise. Lockup eliminates this loss by mechanically coupling the engine to the transmission. Street-driven cars with high-stall converters benefit greatly from a lockup kit — recovering fuel economy and reducing fluid temperature by 20–40°F at highway speed.
These are the next calculator pages most likely to be useful once you have this result in hand.
Drivetrain
01Speed and rpm relationships through tire, gear, and transmission ratio.
Drivetrain
02Tire diameter and indicated-speed error between two tire sizes.
Performance
03Quarter-mile ET and trap speed from power-to-weight.
Why torque converter stall speed must match the engine's torque curve shaped by displacement.
Ensure the correct horsepower value feeds your torque converter selection.
How stroker builds shift the torque curve and may require a different stall speed.
It estimates theoretical rpm and converter slip from values such as trap rpm, vehicle speed (mph), and tire diameter (in).
Start with trap rpm, vehicle speed (mph), and tire diameter (in) because those are the core values that move theoretical rpm 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 Gear Ratio & RPM Calculator, Tire Size and Speedometer Error Calculator, and Quarter Mile ET & Trap Speed Calculator so the rest of the combination stays aligned.
