Tire Size Calculator
Plus-Size, Lift, Off-Road.
Compare your OEM tire to a new size. Get the diameter difference, how far off your speedometer will read, and how revolutions-per-mile change. Essential before plus-sizing, installing a lift kit, or going up to 33s.
Reading a tire size
A tire labeled 265/70R17 means:
- 265 — section width in millimeters (tread width plus sidewalls)
- 70 — sidewall aspect ratio (70% of 265 mm = ~185 mm sidewall)
- R17 — radial construction, fits a 17-inch wheel
When the calculator matters
The 3% rule — why staying close to OEM matters
Stay within 3% of OEM diameter to avoid speedometer recalibration, ABS / traction-control sensor issues, and transmission-shift schedule drift. Beyond 3%, expect to need a handheld tuner to correct the speedo (AEV ProCal for Jeep, SCT for Ford, Superchips for GM).
Fender clearance — what changes with bigger tires
Larger diameter gains ground clearance at the differential (half the diameter delta), but sometimes rubs on uptravel. Trim the front lower valance or add a leveling kit if rubbing occurs at full lock with compression.
Load rating — going wider safely
If going wider, check the new tire's load index matches or exceeds the OEM spec on the door sticker. Under-rated tires fail under tow load.
How tire size affects your speedometer and odometer
Your speedometer and odometer are calibrated to a specific tire diameter at the factory. When a transmission output shaft turns a known number of revolutions per minute, the car assumes the tire makes a known number of revolutions per mile, and the dash reads a speed based on that math. Install a larger tire and each revolution covers more pavement, so the real speed is higher than what the dash shows. Install a smaller tire and the opposite happens — the dash over-reads and the odometer over-counts. That matters for tickets, lease mileage limits, warranty, and resale.
The math is simple: percent diameter change equals percent speedometer error. A tire that is 3% larger than OEM means the dash reads 3% low. At an indicated 65 mph, true speed is about 67 mph. A 3% odometer under-read on a 36,000-mile lease adds up to 1,080 unrecorded miles — which the dealer will catch at turn-in.
Modern vehicles also use wheel-speed sensors for ABS, traction control, stability control, and in many cases transmission shift scheduling. Stay within about 3% diameter of OEM and the systems adjust within their built-in tolerance. Go further and you may trigger false ABS warnings, unstable shift points, or traction control that cuts in at the wrong moments. Above 5% change you usually need a handheld tuner.
Frequently asked questions
What is plus-sizing and how much can I safely plus?
Plus-sizing means going up in wheel diameter while dropping sidewall height to keep overall diameter about the same. Plus 1 and plus 2 are common on most sedans; plus 3 is the upper limit for ride quality. As long as overall diameter stays within 3% of OEM, your speedometer and ABS sensors remain accurate.
How much speedometer error is legal?
Federal tolerance is 5 mph at 50 mph (roughly 10%). Most states enforce within that range. Going 3% over OEM diameter produces about 2 mph error at 65 mph — well within legal bounds, but it also means your odometer under-reads by 3%, which matters for warranty and lease mileage.
What causes tire-to-fender rubbing after I upsize?
Rubbing usually comes from width, not diameter — the tire contacts the inner fender liner or strut at full steering lock with suspension compressed. Fixes: roll the fender lip, trim the front lower valance, add a leveling kit for a half-inch of lift, or step down to a narrower tire with the same overall diameter.
Why does my truck's load rating matter when I upsize?
Load index on the tire sidewall must meet or exceed the door sticker spec. Under-rated tires fail catastrophically under load — especially towing or hauling. A 265/70R17 SL (standard load) cannot safely replace an E-rated (10-ply) OEM tire on a half-ton truck even though the size looks identical on paper.
Do I need different wheels or just different offset?
Wheel offset determines how far in or out the wheel sits. Aftermarket wheels with less positive offset (or negative) push the tire outward, adding width to the outside. Going too far can bind on fenders, stress wheel bearings, and void warranties. Stay within 5 mm of OEM offset unless you have measured clearance at full articulation.
Should winter tires be the same size as my summer tires?
Narrower is often better for winter. A narrower tire concentrates weight into a smaller contact patch, cutting through snow to reach pavement. Many drivers run winter tires one size down from summer on a dedicated steel wheel. Keep overall diameter within 3% of OEM so ABS and stability control calibration stays correct.
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