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upsizing wheels and tires: what changes and what to watch

bigger wheels look better on most vehicles. they also change how the car drives, and a few things people do not expect.

keep the rolling diameter the same

this is the rule that governs everything else. going up an inch in wheel means going down in tire sidewall to keep the overall diameter within about 3% of standard, which is called plus sizing.

get that wrong and:

  • your speedometer and odometer read wrong. a larger overall diameter means you are going faster than the dash says, and adding fewer kilometres than you actually drive.
  • the ABS and traction control work from wheel speeds and are now being fed a wrong assumption.
  • on some vehicles the transmission shift points move, because they are based on road speed.

what you gain and what you give up

gain: sharper steering response, more grip in the dry, and a better stance.

give up: ride quality, because a shorter sidewall has less air to absorb a pothole. wheel damage becomes far more likely for the same reason. unsprung weight usually goes up, which slightly hurts acceleration, braking and fuel economy. and a wider, lower-profile tire is generally worse in snow and slush.

the Ontario considerations people forget

pothole season. a 40-series sidewall gives your rim very little protection, and a bent rim is a new wheel rather than a repair on most alloys.

winter. if you are upsizing for summer, consider keeping the original smaller wheels for winter tires. narrower is better in snow, and you are not risking the good rims on salted roads.

the TPMS sensors need to move to the new wheels or be duplicated, and on some vehicles they have to be programmed to the car.

what we will tell you

whether the size you want clears the brakes and the suspension at full lock, what it does to your speedometer, and whether the load rating is adequate for the vehicle. that last one gets overlooked and matters on trucks and SUVs.