READ A SIDEWALL, MEASURE IT, AND COMPARE ONE SIZE AGAINST ANOTHER
Tire sizing
Tire Size CalculatorOverall diameter, sidewall, circumference and revs per mile from any metric or flotation size.→Tire Size ComparisonTwo sizes, scale drawing, diameter delta, clearance change and speedometer error.→Tire Diameter CalculatorDiameter from a size, and — the useful direction — real sizes that hit a target height.→Tire Circumference CalculatorRolling distance per revolution and revs per mile, the figures the ECU actually uses.→Tire Revolutions Per Mile CalculatorRevolutions per mile and per kilometre from any tire size.→Tire Load Index CalculatorLoad index decoded to lbs/kg, or the index a target load needs.→Tire Speed Rating DecoderEvery speed rating letter decoded to mph and km/h.→Tire DOT Date & Age CalculatorDOT code decoded to a manufacture date, plus how old that makes it.→Tire Size ConverterOne size, shown in metric, flotation and inches simultaneously.→Tire Aspect Ratio CalculatorAspect ratio from a real sidewall measurement, or the reverse.→
WHAT A SIZE CHANGE DOES TO ENGINE SPEED, INDICATED SPEED AND MILEAGE
Drivetrain and speed
Speedometer Error CalculatorTrue speed against indicated speed after a tire change, plus odometer drift.→Gear Ratio CalculatorCruising RPM from axle ratio and tire size, plus the regear that restores stock behaviour.→RPM and Speed CalculatorRPM at any speed and speed at any RPM, gear by gear, with overall ratios.→Final Drive / Differential Ratio CalculatorAxle ratio from ring and pinion tooth counts, the way it is actually defined.→Transmission Gear Ratio CalculatorOne gear's ratio from a lift-test RPM reading, input against output.→Overall Gear Ratio CalculatorThe full chain multiplied together, transfer case included.→Regear CalculatorStart from the RPM you want and solve for the ratio that gets you there.→
OFFSET, BACKSPACING, POKE AND INBOARD CLEARANCE
Wheel fitment
Wheel Offset CalculatorHow far a new offset pokes out or tucks in, with backspacing conversion.→Wheel Backspacing CalculatorBackspacing to offset and back, with the clearance consequence of each change.→Bolt Pattern CalculatorPCD in both units, measured from your own wheel, with the close-but-wrong patterns flagged.→Turning Radius & Turning Circle CalculatorCentreline and curb-to-curb turning radius, plus the full turning circle.→Wheel Width CalculatorThe approved rim-width band for any tire section width.→Wheel Fitment CalculatorTire edge position from wheel width, offset and tire section width together.→Fender Clearance CalculatorRemaining fender clearance from a baseline measurement and a size change.→Suspension Clearance CalculatorRemaining inner (strut/control arm) clearance after a fitment change.→
DISPLACEMENT, COMPRESSION AND THE RELATIONSHIP BETWEEN POWER AND TORQUE
Engine building
Engine Displacement CalculatorCubic inches, cc and litres from bore and stroke, plus overbore steps.→Compression Ratio CalculatorStatic compression from every volume above the piston, itemised.→HP, Torque & RPM CalculatorSolve any one of horsepower, torque or RPM from the other two.→Piston Speed CalculatorMean piston speed from stroke and RPM — the real limit on a long-stroke engine.→Bore & Stroke CalculatorWork backwards from a target displacement to the bore or stroke it needs.→Cylinder Volume CalculatorSwept, clearance and total volume per cylinder, from bore, stroke and CR.→Volumetric Efficiency CalculatorVE from actual airflow against the theoretical 100% figure.→
WHAT THE VEHICLE ACTUALLY COSTS TO FUEL, PER MILE AND PER YEAR
Running costs
READING WHAT A VEHICLE ACTUALLY IS, FROM THE NUMBER STAMPED ON IT
Vehicle identity
WHAT THE NUMBERS PREDICT ABOUT ACCELERATION AND FORCE AT THE ROAD
Performance estimates
Power-to-Weight Calculatorhp per ton, pounds per hp, W/kg and kW/tonne, with what each figure implies.→Quarter Mile & 0–60 CalculatorET and trap speed from the drag-strip formulas, plus an honest 0–60 estimate.→Wheel Torque CalculatorEngine torque multiplied through the drivetrain, and the force it produces.→
TORQUE AND PRESSURE BETWEEN THE UNITS SPECIFICATIONS ACTUALLY USE
Unit conversion
Torque ConverterNm, lb·ft, kgf·m and lb·in, with a note on how much rounding a spec can take.→Tire Pressure Converterpsi, bar, kPa and more — plus why the placard figure is a cold pressure.→PSI to Bar Converterpsi and bar, either direction, with common tire pressures already worked out.→PSI to kPa Converterpsi and kPa, either direction, matched against common placard values.→MPG to L/100km ConverterMPG and L/100km, either direction, with the inverse-scale trap explained.→HP to kW Converterhp, kW and PS together, with the three-way mix-up explained.→
AIR-FUEL MIXTURE, INJECTOR SIZING AND FUEL DELIVERY
Fueling
Air-Fuel Ratio & Lambda CalculatorAFR and lambda together, across eight fuels with real stoichiometric figures.→Fuel Injector Size & Duty Cycle CalculatorSolve for injector size, or check the duty cycle of injectors you already have.→Fuel Pump Size CalculatorRequired pump flow in LPH from target horsepower and fuel type.→
BOOST PRESSURE, COMPRESSOR MAPS AND CHARGE COOLING
Forced induction
THE HYDRAULIC CHAIN FROM PEDAL TO ROTOR, BIAS AND STOPPING FORCE
Brakes
Brake Pressure CalculatorThe full chain: pedal force to line pressure to clamp force to torque.→Brake Bias CalculatorFront/rear bias from actual torque, against a weight-based baseline.→Braking Force CalculatorBraking force from target g-force, with an idealized stopping distance.→Brake Rotor Size CalculatorKinetic energy per stop, plus sizing guidance by use case.→
SPRING RATE, TRAVEL AND THE ANGLES A WHEEL ACTUALLY SITS AT
Suspension and alignment
Suspension Spring Rate CalculatorSpring rate, wheel rate and natural frequency, solved either direction.→Ride Height CalculatorRide height change from spring length change, via motion ratio.→Wheel Travel CalculatorWheel travel from shock travel, or the reverse, via motion ratio.→Camber CalculatorCamber angle from a measured offset, no gauge required.→Caster CalculatorCaster angle from a two-position sweep measurement.→Toe Angle CalculatorToe distance to angle and back, using tire diameter.→
THREAD SIZE, PITCH AND TORQUE ACROSS METRIC AND IMPERIAL
Fasteners
Bolt & Thread Size ConverterMetric and imperial bolt sizes side by side, with standard thread pitches.→Thread Pitch CalculatorPitch to TPI and back, plus lead angle and pitch diameter.→Torque Angle CalculatorThe clamp travel a torque-to-yield angle spec actually produces.→Bolt Torque CalculatorStarting torque from bolt grade, size and friction condition.→
ALREADY WORKED OUT
Compare any two sizes →Popular tire size comparisons
195/65R15vs205/55R16195/65R15vs205/60R16195/65R15vs215/60R16195/65R15vs215/55R17195/65R15vs225/50R17195/65R15vs235/45R17195/65R15vs235/40R18195/65R15vs245/40R18205/55R16vs205/60R16205/55R16vs215/60R16205/55R16vs215/55R17205/55R16vs225/45R17205/55R16vs225/50R17205/55R16vs235/45R17205/55R16vs235/40R18205/55R16vs245/40R18205/60R16vs215/60R16205/60R16vs215/55R17205/60R16vs225/45R17205/60R16vs225/50R17205/60R16vs235/45R17205/60R16vs235/40R18215/60R16vs215/55R17215/60R16vs225/45R17215/60R16vs225/50R17215/60R16vs235/45R17215/60R16vs235/40R18215/60R16vs245/40R18215/55R17vs225/45R17215/55R17vs225/50R17
HOW THESE ARE BUILT
Why the arithmetic is on the page
NOTHING HIDDEN
Every tool publishes the expressions it evaluates. If you would rather check a result by hand, or explain it to someone at a parts counter, the working is there to follow rather than something to take on trust.
NOMINAL, NOT MEASURED
Tire figures are calculated from the size marked on the sidewall. Real tires vary from nominal by a few millimetres between manufacturers, and more as tread wears. These numbers plan a job; a tape measure finishes it.
!
Plan with these, verify on the vehicle.Calculated clearance is not measured clearance. Before committing to a tight fitment, check the actual gaps at full steering lock and full suspension compression, and confirm load ratings against the vehicle's own tire placard.