Tire Size Comparison Calculator

Your speedometer and odometer are calibrated to the rolling circumference of the factory tire (78.45 in per revolution). Moving to 235/40R18 (79.80 in circumference) changes revolutions per mile from 807.6 to 794.0 — a -13.6 rev/mi shift. At 60 mph indicated on your cluster, true road speed becomes approximately 61.0 mph (+1.72% error). That +1.72% variance sits within the ±2–3% band most OEMs target for speedometer accuracy, so daily driving and typical enforcement margins are usually unaffected. Odometer distance will also skew proportionally: over 10,000 miles, a 1.7% error accumulates to roughly 172 miles of discrepancy versus actual distance traveled.

The dimensional delta is moderate — +0.43 in in diameter and +0.39 in in section width — so many vehicles with healthy factory clearance margins can accept this swap without modification. Still verify at full steering lock and under maximum suspension compression; even modest growth can contact the pinch weld or liner on tightly packaged platforms (especially performance sedans and lowered trucks). Wheel offset and backspacing matter as much as tire size. Key contact points to inspect: front inner fender liner at full lock, rear quarter panel lip under load, pinch weld on the unibody rail, and the leading edge of the rear bumper cutout on short-bed trucks. A wider tire (+0.39 in, +4.44%) increases scrub radius slightly, which can add steering effort and transmit more road noise through the rack. If your vehicle uses adaptive cruise, lane-keep, or automatic emergency braking, confirm that radar and camera calibrations are unaffected — some systems are sensitive to ride-height changes.

The +0.21 in change in static ride height is small enough that most factory-height vehicles can absorb it without a lift kit. You still gain the full diameter benefit for obstacle clearance and break-over angle — just confirm that the larger tire does not contact the fender, liner, or control arms at full suspension travel before relying on the extra clearance off-road. Static ground clearance changes by +0.21 in because overall diameter shifts +0.43 in (+1.72%). That half-diameter rule applies at each axle: a 0.43" diameter change adds roughly 0.21" of clearance under the differential and rocker panels. Approach and departure angles improve proportionally, which matters for off-road and steep driveway transitions. For street-only vehicles, the priority is avoiding contact at full compression rather than maximizing lift height.

Revs per mile decrease by -14 (-1.69%), lowering highway RPM by about 14 at 60 mph (808 → 794 RPM). That can slightly improve fuel economy on long highway runs by reducing engine friction. However, a wider or heavier tire increases rolling resistance and rotational mass, which may partially offset the gain — net economy change is typically 0–2% either direction unless the width step is large. Rolling circumference changed +1.35 in (+1.72%), which is the primary driver of cruising RPM. Sidewall height changed -0.29 in — a shorter sidewall reduces flex and heat buildup at speed but transmits more road harshness. For the most accurate estimate, track a full tank before and after the swap on your regular commute.

These sizes use different wheel diameters (17" vs 18"), so factory wheels from 225/45R17 cannot mount 235/40R18. Plus-sizing or minus-sizing requires a complete wheel set matched to the new bead seat diameter, correct hub bore, and load rating. The 18" wheel also changes brake clearance geometry — always confirm caliper-to-wheel clearance before purchase.

Overall diameter increases +0.43 in (+1.72%), from 24.97" to 25.40". Static ground clearance at the lowest point (typically the differential pumpkin or exhaust crossmember) rises by approximately +0.21 in — half the diameter delta. Break-over angle improves because the contact patches move farther from the center of the vehicle, reducing the likelihood of high-centering on obstacles. The larger diameter raises ride height and mildly reduces effective gearing at the wheels; treat it as a stance and clearance change rather than an off-road upgrade.

Modern ABS and ESC systems compare wheel-speed sensor inputs across all four corners to detect slip. A -13.6 rev/mi change alters the expected wheel-speed ratio at any given road speed by -1.69%. At this magnitude, most factory ABS/ESC modules tolerate the variance without fault codes, though a brief relearn drive cycle (10–15 minutes of mixed driving) helps the system establish new baselines. Traction control and hill-descent systems use the same wheel-speed data, so the same tolerance applies. If your vehicle has tire-pressure monitoring, confirm the new size is within the TPMS relearn parameters for your module.

Sidewall height moves from 3.99" to 3.70" (-0.29", -7.16%). Sidewall height is similar, so ride/handling character should remain largely unchanged. Section width changes +0.39 in (+4.44%), which widens the contact patch for improved dry grip and braking but increases hydroplaning speed threshold sensitivity and steering effort. Match tire compound and tread pattern to your climate and driving style for best results.