Garage calculators
Work through tire labels, drivetrain ratios and fuel-mix arithmetic without turning a geometric result into a compatibility or safety claim.
Calculators in this section
Calculate theoretical diameter and circumference.
Tire comparisonCompare the geometry of two metric tire labels.
Speedometer errorEstimate geometric speed change from tire diameter.
Gear ratio & RPMFind theoretical engine RPM from gearing and tire size.
Two-stroke mixCalculate oil from a manufacturer-specified ratio.
Wheel offsetCompare inner clearance and outer wheel position.
Compression ratioCalculate static ratio from cylinder and clearance geometry.
Engine displacementCalculate total displacement from bore, stroke and cylinders.
Power to weightCompare horsepower and vehicle weight in common units.
Sprocket ratioFind chain-drive ratio and driven sprocket RPM.
Injector sizeEstimate per-cylinder fuel flow from explicit assumptions.
Theoretical relationships stay labeled
Tire dimensions come from the size label and drivetrain results assume no slip. Real measurements and manufacturer tolerances can differ.
No fitment or operating verdicts
These tools do not approve a wheel, tire, speed, RPM or mixture. Vehicle, tire and equipment manufacturers remain the authoritative sources.
Three groups of pages
Wheels and tires come first, because most jobs start there. The tire size calculator turns a label such as 225/45R17 into a diameter, a circumference and revolutions per mile; the comparison page sets two labels against each other as a percentage; the speedometer page estimates what a diameter change does to the reading; and the wheel offset page shows how much a different width and offset moves the rim inwards and outwards.
Engine geometry is the second group: displacement from bore, stroke and cylinder count, and static compression ratio from the chamber, gasket, deck clearance and piston volume. The third is drivetrain and fuelling: engine RPM against road speed through the gear ratio and final drive, chain-drive ratios from sprocket teeth, injector flow from a power target, power-to-weight for comparing cars, and the two-stroke mix calculator for the ratio your equipment manual specifies.
Frequently asked questions
Does a small tire-size difference mean the tire fits?
No. Fitment also depends on wheel width and offset, clearance at full lock and full suspension travel, load and speed ratings, and how the vehicle’s ABS and stability systems were calibrated. The placard on the door frame and the manufacturer’s approved list decide it.
Does the mix calculator choose the correct ratio?
No. Enter only the ratio specified by the equipment manufacturer. The calculator works out how much oil that ratio needs for the fuel you are mixing; it cannot tell you what your engine wants.
Which tools go together?
The tire size calculator feeds the gear ratio RPM calculator with a circumference, and the comparison and speedometer pages with a diameter. Bore and stroke feed both the displacement and the compression ratio pages.
Are these figures good enough to build an engine on?
They are good enough to plan with and to sanity-check a supplier’s numbers. Every result here is theoretical geometry from the figures you type, so measured components, manufacturer tolerances and a builder’s judgement still decide the build.