Tire Size Comparison Calculator

Mount one tire on the intended wheel and install it at the corner that typically rubs first on your platform — often the front driver side on lowered or wide-track vehicles. Turn the steering to full lock in both directions while watching the gap between the tire shoulder and inner fender liner, pinch weld, and strut. Have an assistant bounce that corner through full suspension compression while you check for contact at the liner, control arm, and brake line. Even moderate size changes can rub on tightly packaged platforms — do not skip the rear corners on independent rear suspension vehicles.

The speedometer error on this comparison sits within many OEM tolerance bands, but verify against a GPS or known speed trap on your first highway drive. Dealer scan tools, manufacturer apps, and platform-specific tuners (FORScan on Ford, HP Tuners, etc.) can apply a tire-size correction factor where supported. Aftermarket speedometer correction modules and some aftermarket clusters accept a rolling-circumference input directly. Odometer distance accumulates the same proportional error as the speedometer — factor that into lease mileage or maintenance-interval tracking if you rely on the cluster counter.

255/35R19 mounts on a 19" wheel while 245/40R18 requires 18" — the bead seat diameter differs, so factory wheels from the current size cannot mount the new tire. Plan on a complete wheel set with correct hub bore, load rating, and brake caliper clearance for the larger or smaller rim. Plus-sizing and minus-sizing also change the brake rotor-to-wheel-barrel relationship — confirm caliper clearance before purchase.

Many factory-height vehicles absorb moderate size changes without a lift kit, but clearance depends on your exact wheel offset, suspension travel, and fender shape — not tire math alone. Static ride height is only half the picture: the tire moves through an arc as the suspension compresses and the steering turns. Always verify at full droop and full compression. Break-over and approach angles improve when diameter grows, but only if the tire clears the fender at maximum compression — contact at full travel negates the clearance gain.

ABS and stability-control modules compare wheel-speed sensor inputs across all four corners. A tire with a different rolling circumference changes the expected speed ratio at any given road speed. The revolutions-per-mile shift on this comparison is within the tolerance band most factory ABS modules accept without fault codes, though a 10–15 minute mixed driving relearn cycle helps establish new baselines. Traction control, hill-descent, and adaptive cruise systems use the same wheel-speed data — the same tolerance applies. Confirm your TPMS module supports the new size and that sensors are relearned after mounting. Some modules require a dealer tool; others relearn after a drive cycle at specified speeds.

Mixing significantly different rolling circumferences across an axle — or between front and rear on AWD platforms — can stress differentials and confuse traction systems. The recommended approach is to replace all four tires at once when overall diameter changes meaningfully, so every corner reports a consistent wheel speed to ABS and AWD controllers. If budget requires a staggered approach, keep the most worn tires on the same axle and never mix bias-ply with radial or widely different tread depths on AWD vehicles. After installing four matching tires, rotate on the schedule in your owner's manual and recheck inflation cold — mismatched pressure mimics mismatched diameter.

Highway fuel use follows two tire-driven variables: cruising engine speed and rolling resistance. Both shift when overall diameter, sidewall height, or section width change — the Performance & Driving Impact section covers the directional effect on your setup. With cruising RPM nearly unchanged, fuel economy should track the reference tire closely — tire construction, tread compound, and inflation pressure drive most real-world differences at this point. Track a full tank on your regular commute before and after the swap. Calculated dimensions predict the direction of change, not an exact MPG figure — your driving style, terrain, and vehicle load dominate the outcome.

The taller sidewall on 245/40R18 generally absorbs more vertical impact before the rim sees load, which softens the ride — but can feel less precise during quick direction changes. Tire pressure matters as much as geometry: even a correctly sized tire feels harsh when over-inflated or vague when under-inflated. Reset to the placard cold pressure after mounting and recheck after the first hundred miles. Suspension bushings, shock condition, and alignment settings amplify or mask tire changes. If the vehicle pulls, tram-lines on grooved pavement, or shows uneven wear after the swap, schedule an alignment — especially when section width or wheel offset changed. For winter or all-season compounds, tread block design and siping influence noise and wet grip independently of the size label — compare UTQG traction ratings when choosing between brands at the same size.