Automotive Calculators. Sixteen free tools for performance car math: tire sizes and speedometer error, wheel offset and poke, gearing and rpm, quarter mile ET and trap speed, drag-limited top speed, race fuel and stint planning, horsepower from a time slip, torque and horsepower at any rpm, engine displacement, carb sizing, injector sizing, fuel pump sizing, AFR and lambda, corner weights and cross weight, compression ratios, and boost. Every formula is validated against a published reference and shows its real-world accuracy - an estimate with its error bars beats a number pretending to be exact. All of it free: no ads, no accounts, no premium tier, no catch.

Tire Size Calculator

Compare two tire sizes: diameter, sidewall, circumference, and revs per mile, drawn to scale, with the speedometer and odometer error the swap causes.

Gear Ratio Calculator

Rpm in every gear at your cruise speed, shift-drop rpm, speed per 1,000 rpm, and top speed at redline, on an interactive rpm vs speed chart.

Horsepower from Trap Speed

Four published formulas turn a quarter mile trap speed into horsepower, each shown with its real accuracy, plus a launch check that reads your ET.

Fuel Injector Size Calculator

Lb/hr and cc/min per injector from power, BSFC, and duty cycle. E85 sizing, fuel pressure correction, and a reverse check on the injectors you already have.

Compression Ratio Calculator

Static and dynamic compression from the full volume budget, with quench distance, displacement, and a solver for the chamber volume your target ratio needs.

Boost to Horsepower

What a turbo or supercharger could make from your NA baseline, with the uncertainty shown instead of hidden, and the boost needed for a target power.

Quarter Mile Calculator

ET and trap speed from power and weight via four published formulas with their real accuracy, plus an eighth mile conversion that shows the honest factor range.

Wheel Offset Calculator

Compare two wheels on the same hub face, drawn to scale: poke at the fender, clearance at the strut, backspacing, and an optional spacer folded in.

Engine Displacement Calculator

Bore, stroke and cylinders to cc, liters and cubic inches, with a to-scale cylinder drawing, rod ratio, mean piston speed in context, and common overbores.

Carb CFM Calculator

Displacement and rpm to the carburetor rating to buy, as an honest range across volumetric efficiency, with two-barrel vs four-barrel ratings handled correctly.

Horsepower Torque Calculator

Convert between hp, kW or PS and lb-ft or Nm at any rpm, solve for whichever value is missing, and see why every dyno graph crosses at 5252 rpm.

Top Speed Calculator

Drag-limited top speed from power, CdA and weight via the road-load equation, with cited body-shape presets and the honest CdA sensitivity band.

Race Fuel Calculator

Fuel for a race or stint from lap time and burn per lap - stints, pit stops and margins for iRacing, ACC and track days, with every lap itemized.

AFR Lambda Calculator

Convert AFR to lambda and back for seven fuels, with custom ethanol blends, a lambda scale drawing, and the same mixture shown in every fuel.

Fuel Pump Calculator

Required pump flow in lph and gph from power and BSFC, at the rail pressure boost really creates - read the pump chart there, not the headline number.

Corner Weight Calculator

Four scale readings become total, front, rear, left, right and cross weight percent, drawn on a top-view diagram of the car with both diagonals.