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. This comparison involves a large dimensional step — repeat the check at the rear, where the tire arc can contact the quarter panel lip under load.

The speedometer error on this comparison exceeds the ±2–3% band most OEMs target, so recalibration is worth planning before long-term use. 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.

Both sizes use a 18" bead seat, so your existing wheels may work if the internal barrel width supports the new section width and the offset positions the tire correctly in the well. As a rule of thumb, each 10 mm of additional tire width typically needs roughly 5 mm of additional wheel width. Confirm the wheel load rating meets or exceeds the new tire's load index and that the hub bore and lug pattern match your hub.

Large dimensional steps enlarge the tire envelope in every direction. Some vehicles need a mild lift, revised offset, or minor trimming even when the diameter change looks modest on paper. 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 exceeds the ±3% wheel-speed tolerance cited by many OEMs — a brief fault code or reduced intervention is possible until the system relearns or is recalibrated. 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. When cruising RPM rises, the engine turns more revolutions per mile at the same indicated speed, which usually increases highway fuel load. A taller or softer sidewall can add rolling resistance from flex even when overall diameter is unchanged. 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 shorter sidewall on 235/40R18 generally firms transient response and sharpens turn-in on smooth pavement, but transmits more harshness over expansion joints and potholes. 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.