Why choosing wheels and tire sizes can make a big difference to you

Toronto Star

17-05-2025

There are few easier ways to change the look of your car or truck than putting on a different set of wheels.
There are a surprising number of factors that need to be considered, even at the simplest level of 'will they bolt on?'
The changes can change how your vehicle steers, rides, or handles, appreciably — for better or for worse.
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This is the case if you deviate from what the manufacturer equipped your model with. Generally, the larger the wheel, the shorter the sidewall, the firmer the ride.
Some vehicle models — this applies to pickup trucks, in particular — are available with a wide variety of wheel and tire sizes, and typically, any of the larger sizes will bolt directly onto and have sufficient clearances on models not originally equipped with them. Take, for example, Ford's F-150: base models come with 17-inch wheels and tires. Some models are available with 18-inch wheels, many come with 20-inch wheels, and the top trims offer 22-inch wheels. The wheels themselves would interchange, although there are minor differences in the outside diameter of the original tire sizes used.
Not everyone wants to use a factory wheel for their upgrade. If an appreciable number of examples of that model already wear them, what's the point? What to do?
The aftermarket provides a greater selection of options, but this search requires diligence. Most aftermarket wheels are designed to fit many vehicles and often come in a range of widths and offsets — the offset is how far in or out the wheel/tire sits relative to the body and suspension — to suit buyers' preferences — which don't always line up with factory choices.
Beyond offsets, the width of the tires and wheels has an important role to play. Shopping for a 15-inch wheel for the recently acquired 1991 Mazda Miata of my wife, Barb, revealed factory-original and aftermarket choices. Some early MX-5/Miatas did come with 15-inch wheels that would directly fit. None offered the look Barb desired, and a clean used set were comparable in price to some new aftermarket sets, so aftermarket won.
Often, when moving up a wheel diameter — known as 'plus-one' sizing — you'd increase the tread width of the tire, and in the Miata's original sizes, this is the case: from a 185 (14-inch) to a 195 (15-inch) section width. (This is not exactly tread width, but it is the industry's chosen measurement standard.)
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In the Miata's optional 195/50R15 size, tire choices seemed to be either race compound tires with a fruit-fly lifespan and aquaphobia, or tires best suited for an aging minivan — just what we'd wanted to avoid.
Moving to a 205/50R15 resulted in a livable 1.8-per-cent increase in overall diameter, for a speedometer error of just 2 km/h at 100 km/h, and opened up far more options. Based on positive reviews, we settled on the still very performance-skewed Falken Azenis RT615K+.
Falken's specs showed an approved rim width range of 5.5-7.5 inches. ('Rim' is the part of the wheel where the tire mounts.) Research, coupled with our desire to stay close to stock in order to avoid clearance issues in the Miata's apparently cosy wheel wells, prescribed a maximum seven-inch width.
Fortunately, some 15x7-inch wheels with the correct bolt pattern, in a close-to-original offset, were available; Konig's lightweight 'Heliogram' wheels got the nod.
So what would happen if we'd found a 5.5-inch or 7.5-inch wide wheel?
Clearance aside, would it make a difference?
Yes.
The Azenis's 'measured rim width' is 6.5 inches. That's the ideal rim width, the one where the tire's section width matches its specs. Per an industry rule-of-thumb, each half-inch in rim width changes the section width by two 10ths of an inch — approximately five mm. With our 205 example, on a 5.5-inch wheel, it'd decrease to 190 mm; on a 7.5-inch wheel, increase to 215 mm, with corresponding changes to actual tread width at the pavement.
So what?
Well, not only does this difference change the size and shape of the contact patch, the hand-sized area where your tire actually touches the road surface, and where all traction occurs, it also changes the relationship between the tire sidewalls and wheels.
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While on your average pickup or crossover these differences would have a lesser impact, a 2022 test by tire retailer Tire Rack of nine wheel/tire width combinations on identical Subaru BRZs showed that choosing the proper wheel width can be the difference between clumsy handling and slow-reacting steering, and crisp, precise responses plus higher limits.
It can also influence the quality of the ride, as tires that are stretched to fit a wider wheel may have less sidewall compliance to absorb minor bumps.
Substantial changes in tire diameter — that is those that fall outside of the auto-industry's accepted recommendation of a limit of three per cent, can create issues with gearing, and be detrimental to performance, driveability, mileage, even braking.
Those wishing to 'go big or go home' need to understand that other mechanical changes, may be required — from updating engine management software to replacing differential gear sets and suspension components.
In changing tires and wheels, it may be wise to seek the advice of a professional, so you can enjoy the final result.
Ask a Mechanic is written by Brian Early, a Red Seal-certified Automotive Service Technician. You can send your questions to [e-mail]. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.