Upsizing — swapping out stock wheels for bigger-diameter aftermarket wheels — is all the rage among street tuners.
Some Key Decision Factors
Is it safe? It can be. The key is mounting a tire to the bigger wheel that doesn’t change the overall diameter of the tire-and-wheel combination. That usually means a tire that has a lower profile — that is, a lower sidewall. And that means adding a wider tire.
Clearance also must be maintained. In other words, your tires should not be scraping against the wheel wells or fender liners when you corner or go over bumps. That means a tire that’s wider by 10- 20 mm may work, but more than that may present problems.
Why do people upsize?
Truthfully, it’s for show. Owners who think they will get a sizable performance increase from a bigger wheel and lower-profile tire often are dreaming. The biggest reason is weight: Bigger wheels and wider tires usually add weight, and weight often reduces performance. Upsized wheels and tires also can upset the balance that was engineered into the chassis.
When Car and Driver magazine tested upsized wheels and tires on a VW Golf, road holding improved in every size (Plus 1, 2, 3 and 4), but fuel economy declined in every size. Braking improved in all but the Plus 1, which took longer to stop.
Minor differences in diameter — say, a few 10ths of a percent — will not unduly affect instrumented readings, but the Car and Driver test demonstrated that differences in overall diameter can have an effect. Speedometer error after upsizing ranged from none to -3 mph.
But if you crave dubs on your Chrysler 300 — those are 22-inch rims, for the uninformed — then have at it.
Tire Upsizing Formula
Here’s how to make sure the overall wheel-and-tire diameter will be the same:
For every 1 inch added to wheel diameter:
– Add 10mm to the tire width
– Decrease sidewall height by 5 to 10 percent
Say your stock tire is a 205-55-16. It has a wheel-and-tire diameter of 24.9 inches or the sum of:
– Wheel height 406.4 mm (16 inches)
– Sidewall height 112.75 mm (205mm width x 0.55)
– 2nd Sidewall height 112.75 mm (205mm width x 0.55)
– Total diameter 631.9 mm (24.9 inches)
Using the Plus 1 formula, the next size up would be a 215-17-45 (plus 1 inch for the wheel, plus 10 mm for the tire width, and minus 10 percent on sidewall height). So, it has a similar wheel-and-tire diameter of 24.61 inches:
Wheel height 431.8 mm (17 inches)
– Sidewall height 96.75 mm (215mm width x 0.45)
– 2nd Sidewall height 96.75 mm (215mm width x 0.45)
– Total diameter625.3 mm (24.61 inches)
– To go up more sizes, just apply the formula again.
A couple of cautions:
If you go with extremely low-profile tires, watch the potholes. Road defects will more easily pinch the sidewall on tires with profiles as low as 35, damaging the tire, the wheel or both. And never mount a tire to a wheel that’s wider than the recommended width for the tire.