RPM → MPH
Find theoretical vehicle speed at a given engine RPM.
Theoretical Speed
No slip correction
—
Performance Math Without The Guesswork
Shop Calculators
Fast, practical calculators for LS and GM truck builds. Work backward from the result you want, compare combinations, and sanity-check the numbers before you buy parts or change the tune.
Live Example● Calculating
Engine Speed6,200 RPM
Rear Gear4.10
Tire33.0 IN
4th Gear Speed148 MPH
Core relationship: RPM × tire diameter ÷ (trans ratio × axle ratio × 336) = vehicle speed
DRIVETRAIN
RPM & Road Speed
Use actual transmission ratio, rear gear and tire diameter. These are theoretical values and do not include tire growth or converter/clutch slip.
MPH → RPM
Estimate engine speed at cruise, a shift point or a target road speed.
Theoretical RPM
Useful for cruise planning
—
TIRES & GEARING
Diameter & Effective Ratio
Metric tire sizing changes the effective gearing of the truck. These tools make that change easy to see.
Metric Tire Diameter
Convert a metric tire size into approximate overall diameter.
Approx. Diameter
Circumference: —
—
Effective Gear Ratio
See how a tire-size change alters the gear ratio the truck effectively feels.
Effective Ratio
Larger tire = numerically lower effective gear
—
TRANSMISSION
Shift-Speed Planner
Enter your desired shift RPM and the calculator will show the theoretical road speed for every 6L80 gear. You can overwrite the default ratios for another transmission.
Multi-Gear Shift Speed
Useful for checking whether commanded shift speeds and RPM thresholds agree with each other.
Important: real shift completion occurs after the command and can differ because of shift time, converter slip, tire growth and transmission strategy.
CONVERTER / DRIVELINE
Converter Slip Estimate
Compare actual engine RPM to theoretical zero-slip RPM at the same road speed. Best used under steady conditions with known gear and tire size.
Speed-Based Slip
Positive slip means the engine is turning faster than the zero-slip driveline calculation predicts.
Estimated Slip
Theoretical RPM: —
—
FUEL SYSTEM
Injector Sizing
A quick planning tool using target horsepower, BSFC, injector count and maximum duty cycle. Always leave realistic headroom and account for fuel pressure and fuel type.
Required Injector Flow
Calculate approximate flow per injector in lb/hr.
Minimum Per Injector
At entered BSFC and duty cycle
—
Injector HP Capacity
Estimate supported horsepower from injector flow.
Approx. Fuel-Limited HP
Planning estimate only
—
ENGINE GEOMETRY
Static Compression Ratio
Use bore, stroke, chamber volume, piston volume, gasket and deck clearance to estimate static compression ratio. Enter dish volumes as positive cc and domes as negative cc.
Compression Ratio
All dimensions should represent the actual finished engine combination.
Static Compression Ratio
Clearance volume: —
—
PERFORMANCE ESTIMATE
Quarter-Mile Estimate
A traditional weight-and-power estimate for rough planning. Real trucks vary substantially with traction, gearing, aerodynamics, shift strategy, weather and driver.
ET & Trap Speed
Enter race weight with driver and estimated wheel horsepower for a rough theoretical baseline.
Estimated 1/4 Mile
Trap speed: —
—
This is a rough mathematical estimator, not a prediction of what a specific truck will run.
QUICK REFERENCE
Useful Numbers
A few common constants and reference points used throughout the toolbox.
Speed Constant336
Common RPM/MPH conversion constant when tire diameter is in inches.
Inches per mm25.4
Used to convert metric tire sidewall dimensions into inches.
6L80 4th1.152:1
Common direct-ish comparison gear used in shift-speed calculations.
6L80 6th0.667:1
Overdrive ratio useful for highway cruise RPM planning.
Math Is A Starting Point
These calculators are planning tools. Manufacturing tolerances, actual tire rollout, converter behavior, transmission slip, calibration strategy, fuel system pressure, measurement method and real-world conditions can change the result. Verify critical values on the actual vehicle before making mechanical or calibration decisions.
REFERENCE FORMULAS & DATA
Where The Numbers Come From
RPM / MPH
The standard 336-based relationship is published by TREMEC. For automatics, use transmission ratio × axle ratio as total gear ratio and remember converter/tire slip can move the real number.
See formula and example →6L80 ratios
The default six-speed ratios in the shift planner match GM Upfitter published specifications.
See the GM ratios →Injector sizing
The simple BSFC calculator is a planning estimate. Injector Dynamics warns guessed BSFC can be misleading, so final selection should use validated injector data, fuel type and differential pressure.
Read fuel-system guide →