ElpiAutoElpiAuto
Car math, worked out for you
Free tire and wheel calculators that show the working. Type a size, see every dimension, the effect on your speedometer, and whether a swap actually fits — no account, no upsell.
Calculators
Each one uses the same geometric engine and cites its formulas.
Tire Size Calculator
Type one size and see what the code means — overall diameter, section width, sidewall, circumference and revs per mile, drawn to scale, plus every alternative size that fits your wheel inside the 3% rule.
Open →Two sizesTire Size Comparison
Two sizes side by side — every dimension, the exact percentage difference, the speedometer and clearance effect, and a to-scale drawing, plus the comparisons people run most often.
Open →Metric ↔ inchTire Size Converter
Any size in the other notation — 235/75R15 becomes 28.9×9.3R15 — with every dimension in inches and millimetres, plus conversion charts by wheel and by height.
Open →FitmentWheel Offset Calculator
Compare two wheels by width and offset — how much the new one pokes toward the fender, how close it sits to the strut, and the backspacing change, in a cross-section drawing.
Open →psi ↔ kPa ↔ barTire Pressure Converter
Convert tire pressure between psi, kPa and bar, with a conversion chart for the pressures on a door placard and the rule for setting them right.
Open →mpg ↔ L/100kmFuel Cost Calculator
Turn a trip's distance and the fuel it burned into your real consumption — in US mpg, UK mpg, L/100km and km/L — plus the fuel needed and what it costs, split between passengers.
Open →Speedo & odometerSpeedometer Error Calculator
How far the speedometer and odometer drift when you change tire size — the true speed at each point on the dial, and the odometer error over 10,000 miles.
Open →Stance & fitmentTire Stretch Calculator
How stretched a tire sits on a given wheel width — the stretch percentage, the maker's approved rim range, and whether it holds up for daily driving or show only.
Open →By vehicleBolt Pattern by Vehicle
Look up your car and get its PCD, centre bore and lug thread for every generation, with a checker for whether a wheel will fit.
Open →Bolt Pattern Calculator
Bolt-circle diameter from a stud measurement, and whether one pattern (5x100, 5x114.3) will bolt to another.
SoonTire Diameter Calculator
Overall height from any size, in inches or millimetres, with the revs per mile that follow from it.
SoonHow these are built
The working is on the page
Every result comes with the formula it used and a link to the standard behind it — not a number from a black box.
Metric or inches
One toggle switches every figure — millimetres and km/h, or inches and mph — on all of it, including the diagrams.
No account, no upsell
It is a tool, not a funnel. Nothing to sign up for, nothing pushed at you, no tire shop attached.