Tire Size Comparison Calculator

Your speedometer and odometer are calibrated to the rolling circumference of the factory tire (82.71 in per revolution). Moving to 235/40R18 (79.80 in circumference) changes revolutions per mile from 766.1 to 794.0 — a +27.9 rev/mi shift. At 60 mph indicated on your cluster, true road speed becomes approximately 57.9 mph (-3.51% error). That -3.51% variance exceeds the commonly cited ±3% OEM tolerance. At highway speeds, a 3.5% error can translate to several mph of drift — enough to affect cruise control, navigation ETA logic, and speed-camera margins. Recalibration via dealer tools, FORScan/HP Tuners (where supported), or an aftermarket speedometer correction module is worth considering before committing long-term. Odometer distance will also skew proportionally: over 10,000 miles, a 3.5% error accumulates to roughly 351 miles of discrepancy versus actual distance traveled.

This is an aggressive dimensional step. A -0.93 in diameter increase combined with 0.00 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 (0.00 in, 0.00%) 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 -0.46 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 -0.46 in because overall diameter shifts -0.93 in (-3.51%). That half-diameter rule applies at each axle: a 0.93" diameter change adds roughly 0.46" 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 increase by +28 (+3.64%), meaning the engine turns 28 more RPM at 60 mph (766 → 794 RPM). Higher cruising RPM raises internal friction losses and can trim highway fuel economy by 1–3% in real-world driving, though the effect varies with gearing, aerodynamics, and how much the wider contact patch increases rolling resistance. City driving impact is usually smaller because speeds are lower. Rolling circumference changed -2.91 in (-3.51%), which is the primary driver of cruising RPM. Sidewall height changed -0.46 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.

Both 235/45R18 and 235/40R18 mount on 18" wheels, so your existing rims may work if the internal barrel width and offset support the 235 mm section width. As a rule of thumb, each 10 mm of tire width typically needs ~5 mm of additional wheel width. Verify that the new tire's load index and speed rating meet or exceed your vehicle's requirements, and confirm that the wheel's backspacing positions the bead correctly relative to the hub face — incorrect offset is a leading cause of rubbing even when the tire diameter fits.

Overall diameter increases -0.93 in (-3.51%), from 26.33" to 25.40". Static ground clearance at the lowest point (typically the differential pumpkin or exhaust crossmember) rises by approximately -0.46 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. Note that a smaller diameter reduces clearance and increases break-over vulnerability — confirm this tradeoff aligns with your use case.

Modern ABS and ESC systems compare wheel-speed sensor inputs across all four corners to detect slip. A +27.9 rev/mi change alters the expected wheel-speed ratio at any given road speed by +3.64%. Changes above ~3% can cause the ABS module to flag implausible sensor data, potentially triggering warning lights or reducing intervention effectiveness until the system relearns or is recalibrated. 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 4.16" to 3.70" (-0.46", -11.11%). Shorter sidewalls reduce sidewall flex under cornering load, sharpening turn-in and reducing tread squirm for more precise handling — but they transmit more impact energy to the suspension and are more vulnerable to pinch flats and rim damage on potholes. Section width changes 0.00 in (0.00%), which has minimal effect on contact patch area. Match tire compound and tread pattern to your climate and driving style for best results.