Plus Sizing Guide

Plus sizing is the practice of moving to a larger wheel diameter while reducing the tire’s aspect ratio to keep the overall tire diameter close to unchanged. The 2022+ Subaru WRX is a factory example across three trim levels: the Base ships on 17-inch wheels with 235/45R17 tires, the Premium and Limited step up to 18-inch wheels with 245/40R18 tires, and the TR and tS go to 19-inch wheels with 245/35R19 tires. The wheel diameter incrementally increases, but the overall tire diameter changes by only +1.5% and +1.7% respectively — well within the ±3% threshold where the speedometer reads accurately and the suspension geometry is effectively unchanged. The wheels are larger, the tires are lower-profile, and the handling character changes, but the three trim levels all ride at the same height and spin at virtually the same rate.

This guide covers the logic behind plus sizing, the math for maintaining diameter, the real performance trade-offs, and when it makes sense versus when it does not. If you want to see the numbers for a specific size pairing, the Tire Size Comparison Calculator will show you the diameter difference, speedometer error, and relative sidewall height for any two sizes side by side.

What Plus Sizing Means

If you are unfamiliar with how section width, aspect ratio, and wheel diameter combine to produce overall diameter, How to Read a Tire Sidewall covers the formula. The rest is just math. Overall tire diameter is determined by two things: the wheel diameter, and the total sidewall height added above the rim on both sides. The sidewall height is the tire’s section width multiplied by the aspect ratio percentage. When changing wheel sizes and increasing wheel diameter by 1 inch, you need to reduce the total sidewall contribution by 1 inch (about 12.7mm per side) to maintain the same overall diameter.

Though not frequently heard as much as in the 1990s, the naming convention counts up from the original wheel size: Plus 1 means one inch larger diameter wheel, Plus 2 means two inches larger, and so on. Most passenger car fitments range from Plus 0 (same wheel diameter, wider tire) through Plus 2. Beyond Plus 2, the sidewalls become so short that the practical drawbacks typically outweigh the benefits. Many modern performance cars come with larger diameter wheels from the factory where a Plus 0 is the best choice.

Maintaining Overall Diameter

When upgrading wheels and/or tires, the goal is to keep the new overall diameter within ±3% of the original. Beyond 3%, the speedometer error is large enough to matter, and the change in suspension geometry and fender clearances becomes unpredictable. Most plus sizing choices land within ±1.8%.

The practical approach is to work through the aspect ratio math for the larger wheel diameter and then check a tire size comparison tool or chart to confirm the actual diameter. Please note, theoretical diameter from plugging numbers in to the formula and actual diameter from the manufacturer’s specifications can differ by up to half an inch on performance tires, so be sure to confirm your calculations with real data from the tire manufacturer.

As a working example, the table below is anchored to the WRX Base’s 235/45R17 and shows compatible sizes within ±3%, organized by wheel diameter. Minus 1 is the 16” section (useful for winter sizing), Original is the 17” Base spec, Plus 1 is 18” (the Premium/Limited/GT factory size 245/40R18 appears here at +1.5%), and Plus 2 is 19” (the TR/tS factory size 245/35R19 appears here at +1.7%). All three WRX factory fitments are visible from a single starting point. See the VB WRX Wheel Guide for the full platform context. To find compatible sizes for any starting tire size, use the Tire Size Compatibility Lookup calculator.

Wider tires in the same plus size step introduce a larger diameter change because the wider section width adds more to the sidewall height. Always check the specific combination before committing.

The Trade-Offs

Handling and Steering Response

While many choose plus sizing for aesthetics, it pays off most clearly for performance-oriented drivers. A lower-profile sidewall flexes less under lateral cornering load. The reduction in sidewall flex means the tire transmits steering inputs more directly. The contact patch stays more consistently flat under cornering load rather than rolling onto the sidewall. For a vehicle where the driver wants precise, communicative steering, the improvement is real and noticeable.

Ride Quality

The downside of the same rigidity is that the tire is less forgiving of rough pavement. High-frequency vibrations that a taller sidewall would absorb now pass through to the cabin or even the steering wheel. On smooth roads the difference is minor. On broken urban pavement or chip-sealed rural roads, Plus 2 on a daily driver can produce noticeably harsher ride quality.

Every step of plus sizing trades some ride compliance for steering precision. Plus 1 on most vehicles is imperceptible to most drivers on normal roads. Plus 2 is noticeable on imperfect surfaces. Plus 3 is a meaningful trade that most drivers feel every day unless roads are consistently smooth. Whether that trade is worth it depends entirely on the vehicle, the driver, and the roads. For a dedicated track car, Plus 2 with a stiff suspension tune is a completely rational choice. For a daily driver in a city with aged infrastructure, it may not be.

Wheel and Rim Vulnerability

A taller sidewall acts as a buffer between the rim and a pothole or road edge. When a tire hits a sharp edge, the sidewall compresses, and that compression absorbs energy before the rim takes a direct hit. Lower-profile tires compress less, which means more impact energy reaches the rim. In the absolute worst case, the rim strikes the road surface. The lower the profile, the more likely a significant pothole is to bend or crack a wheel rather than just deform the tire.

Alloy wheels on Plus 2 and Plus 3 fitments in urban environments bend at a meaningfully higher rate than the same wheels on Plus 0 fitments. Forged wheels resist bending better than cast wheels, which is one reason forged wheels are the standard for very low-profile performance fitments. If you are running Plus 3 on cast wheels in a city with rough roads, budget for rim repairs or a set of spare wheels.

Unsprung and Rotational Weight

Wheel weight matters in two ways: as unsprung mass (everything below the suspension that moves with road irregularities) and as rotational mass (everything that spins when the wheel turns). Both affect acceleration, braking, and handling response. Less mass is better for all three of those performance categories.

Larger diameter wheels are typically heavier than smaller diameter wheels of the same design. A set of 19-inch alloys will almost always be heavier than 17-inch alloys in the same wheel line. The larger diameter also means more rotational inertia at the wheel’s outer edge, which amplifies the weight penalty for acceleration and deceleration. On the other hand, plus sizing usually also changes the tire. A lower-profile tire typically weighs less than a taller-profile tire in the same section width. The net weight change depends on the specific combination. If you are moving from heavy OEM cast wheels to lighter aftermarket forged wheels as part of a plus sizing change, the total rotational mass can decrease even while the wheel diameter increases.

If performance is a priority, plus sizing to a lighter flow-formed or forged wheel can be a net gain. Plus sizing to a heavier cast wheel of the same design is a net loss from a weight standpoint, even if the handling character improves.

Speedometer Accuracy

Every combination of tire and wheel has a specific overall diameter, which determines how many times the tire revolves per mile. Your speedometer is calibrated for the original diameter. The ±3% threshold is the practical standard for acceptable speedometer error. Within ±3%, the reading is close enough for regular use as GPS navigation and most local speed limits have more tolerance than that. The Tire Size Comparison Calculator shows the exact speedometer error percentage for any two sizes and flags when you are approaching or exceeding the ±3% threshold.

Cost

Larger diameter wheels cost more than smaller ones of equivalent quality. Larger diameter tires in the same performance category cost more than smaller diameter tires. The cost premium compounds: going from 17” to 19” in a performance all-season category can easily add $100–$200 per tire in addition to the wheel cost premium.

There is also a long-term consideration. Lower-profile tires in a plus-sized fitment typically have shorter tread life than a taller-profile tire. A 35-series tire wears faster than a 50-series tire in most compounds because the reduced sidewall flex means more heat in the tread compound and a smaller compliance buffer for uneven road contact.

When Plus Sizing Makes Sense

You are buying new wheels anyway. If you are replacing a damaged or aesthetically tired set of wheels, choosing a Plus 1 or Plus 2 diameter in a performance category is a natural upgrade with real handling benefits at comparable cost to staying at the original size.

Your vehicle came with a conservative factory fitment. Many economy-focused vehicles ship on 16” or 17” wheels with tall-aspect tires optimized for ride comfort and long tread life rather than handling. Moving to Plus 1 or Plus 2 with a higher performance category tire can substantially improve the driving character of a car, even if it was never meant to be exciting at the factory spec.

You are tracking or autocrossing your car. Plus sizing on a track-oriented build reduces sidewall flex and sharpens steering response. The ride quality penalty is irrelevant when the car is faster.

You want a specific wheel aesthetic. There is nothing wrong with choosing a larger diameter wheel for appearance. Just verify the diameter math before buying and accept the ride trade-off knowingly.

When Plus Sizing Does Not Make Sense

Daily driving in a city with rough roads. The ride quality penalty and wheel vulnerability increase with each plus step. If you drive on broken pavement every day, Plus 2 or Plus 3 will be genuinely unpleasant and will likely lead to expensive wheel repairs.

You are replacing worn tires on a budget. Staying at the OEM size or Plus 0 costs less, preserves ride quality, and avoids the complexity of verifying a new wheel fitment.

Winter driving on dedicated winter tires. Winter tires are almost universally better in smaller diameters. The taller sidewall provides more compliance on packed snow and ice, and the narrower section width cuts through snow more effectively. Minus 1 sizing (one step smaller wheel with a taller tire) is a common and well-supported strategy for winter wheel-tire packages on vehicles that came with large-diameter OEM fitments, provided the smaller wheels clear the brake hardware.

Minus Sizing for Winter

Minus sizing follows the same diameter-maintenance math as plus sizing, in reverse. A WRX Base running 235/45R17 summer tires can run 235/50R16 or 225/55R16 winter tires on 16-inch steel wheels, keeping the overall diameter within 2% of stock. A taller sidewall or narrower treadwidth on a winter fitment is a feature. It provides more compliance, is more resistant to pothole damage on rough winter roads, and allows a narrower section width that cuts better through snow.

Steel wheels are standard for dedicated winter sets because salt and corrosion do less lasting damage to steel than to alloy, and steel is cheaper to replace if that wheel eventually bends. Mounting dedicated winter tires on a separate steel wheel set and swapping them seasonally is typically cheaper over several years than paying a shop to dismount and remount tires twice a year.

You have the diameter math and the trade-offs. If you are working through a specific size combination, use the Tire Size Comparison Calculator to check the exact diameter difference and speedometer error before buying. If you are also changing section width and want to know whether the wider tire fits the new rim, Tire and Wheel Sizing Compatibility has the Tire & Rim Association rim width guidelines.