Downsizing is great for that second set of winter tires. Snow tires tend to become much more expensive at sizes larger than 17”. In addition, the narrower the tire, the more effective it tends to be on snow and ice. So if you have 18” or 19” wheels and want an extra set of wheels for snow tires, it’s often a good idea to go down to 17” or 16” wheels for the winter set, which will give you better winter performance at a lower price.
A Bit of Math
So the math goes like this:
- Multiply the width by the aspect ratio expressed as a decimal. (225 x 0.55 = 123.75)
- Convert the aspect height to inches. (123.75 / 25.4 = 4.87)
- Multiply the aspect height by 2. (4.87 x 2 = 9.74)
- Add the inside diameter of the tire. (16 + 9.74 = 25.74)
- Rinse and repeat for the size of the new tire.
So if I were to upsize to a 17” wheel and keep the width of the tires the same at 225mm, and the aspect ratio the same, the new 225/55/17 tire would have a standing height of 26.74 inches, and my speedometer would be off by more than 2 miles per hour. What I need in order to keep the correct size is a 225/50/17. If I were to also change width, say, from 225 to 245mm, then what I would need is a 245/45/17.
Did I say it was a bit of math? Perhaps it’s just a bit more than a bit. Don’t worry, that’s what computers are for, which is why I use the Tire Size Calculator at Miata.net, or if that doesn't work for you, the one at 1010 tires.com. Plug in the old and new tire sizes and the app will give you the percentage difference between them and tell you what the new size will do to your speedometer readings.
In general, you want the tires to have less than 1% difference. Ideally, you want less than 0.5% difference. Or you can just trust your tire professional, but with the added confidence of knowing what the heck they’re talking about. Knowing is, after all, half the battle. This link will also help it goes the same for the 17 inch rim.
Hoped this helped.Please rate excellent service so I can get Credit for my help thanks. Bonuses greatly appreciated.