Tire Size Comparison Calculator

Your speedometer and odometer are calibrated to the rolling circumference of the factory tire (108.66 in per revolution). Moving to 305/70R18 (109.36 in circumference) changes revolutions per mile from 583.1 to 579.4 — a -3.8 rev/mi shift. At 60 mph indicated on your cluster, true road speed becomes approximately 60.4 mph (+0.65% error). That +0.65% 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 0.6% error accumulates to roughly 65 miles of discrepancy versus actual distance traveled.

This is an aggressive dimensional step. A +0.22 in diameter increase combined with +0.79 in of section width enlarges the tire envelope in every direction — upward into the fender lip, inward toward the strut at full lock, and rearward into the inner liner under compression. Before purchase, cycle the suspension through full droop and full compression (or measure at ride height with a straightedge), check lock-to-lock clearance, and verify that brake caliper and rotor diameter still clear the inner sidewall. Budget for minor trimming, a mild lift, or revised wheel offset if any contact is found. 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 (+0.79 in, +7.02%) 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 +0.11 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 +0.11 in because overall diameter shifts +0.22 in (+0.65%). That half-diameter rule applies at each axle: a 0.22" diameter change adds roughly 0.11" 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 -4 (-0.64%), lowering highway RPM by about 4 at 60 mph (583 → 579 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 +0.71 in (+0.65%), which is the primary driver of cruising RPM. Sidewall height changed +1.11 in — a taller sidewall absorbs more impact energy but adds unsprung mass and flex that can slightly increase rolling resistance. For the most accurate estimate, track a full tank before and after the swap on your regular commute.

These sizes use different wheel diameters (20" vs 18"), so factory wheels from 285/65R20 cannot mount 305/70R18. 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 +0.22 in (+0.65%), from 34.59" to 34.81". Static ground clearance at the lowest point (typically the differential pumpkin or exhaust crossmember) rises by approximately +0.11 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 clearance change is modest; do not expect a dramatic off-road capability shift from diameter alone.

Modern ABS and ESC systems compare wheel-speed sensor inputs across all four corners to detect slip. A -3.8 rev/mi change alters the expected wheel-speed ratio at any given road speed by -0.64%. 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 7.29" to 8.41" (+1.11", +15.25%). Taller sidewalls act as a secondary spring, filtering high-frequency road input and improving comfort on broken pavement — at the cost of slower turn-in response, more body roll in transitions, and reduced steering feedback. Section width changes +0.79 in (+7.02%), 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.