Tires & Wheels Calculators
See how a tire size or wheel offset change carries through to rolling diameter, speedometer error, gear ratio, and contact patch before you buy.
A plus-size wheel and tire swap changes three numbers whether you plan for it or not: rolling diameter, contact patch, and where the assembly sits in the fender. These tires and wheels calculators work out each one from your actual sizes.
From Tire Size to Speedometer Error
Overall tire diameter comes from the rim size plus twice the sidewall height, and sidewall height is the tire’s width times its aspect ratio. Change either number and the rolling diameter changes, but your speedometer keeps counting the same wheel-speed pulses as before.
A taller tire covers more ground per revolution than the factory tire it replaced, so the car is moving faster than the speedometer shows; a shorter tire does the reverse. The actual speed works out to the speedometer reading multiplied by the ratio of new diameter to old diameter.
Factory speedometers are also allowed to read a little high from the factory as a safety margin, but never to read low — so a tire swap that pushes yours into under-reading territory is worth correcting.
Contact Patch, Load, and Pressure
A tire’s contact patch area is approximately its load divided by its inflation pressure. That’s why a heavier vehicle or a heavier load needs more pressure to keep the same contact patch, and why dropping pressure to “get more grip” mostly just lengthens the patch and adds sidewall flex — up to the point where the sidewall itself starts absorbing load it isn’t built for. Wheel offset changes where that contact patch sits relative to the fender and suspension, not how big it is.
Frequently Asked Questions
Why does a plus-size upgrade change my ride height?
Rolling diameter, not rim size, sets ride height. Going up a wheel size while dropping the tire’s aspect ratio to hold the same overall diameter (“plus sizing”) keeps ride height unchanged; increasing overall diameter raises it.
Does lower tire pressure always mean more grip?
Not past a point. Contact patch area grows roughly as pressure drops for a given load, but too little pressure lets the sidewall flex and absorb load instead of the tread, which can shrink usable grip and overheat the tire.
Can a wheel offset change affect my speedometer?
Not by itself. Offset moves the wheel in or out relative to the hub — it changes stance and clearance, not rolling diameter. Speedometer error only comes from a change in overall tire diameter.