Tire & Wheel Fitment Lessons
06-08-2019, 05:59 PM
#1
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Tire & Wheel Fitment Lessons
Learned a great deal recently about tire & wheel fitment when I changed to new wheels and tires. OE on the front was 18 x 8 AMG wheels with 225/40 tires and an ET of 50. According to a website I found, this tire size has a diameter of 25.1 inches. In front I observed that the lower spring perch on the strut was barely a quarter inch above the top of the tire.
The wheels I was putting on came with 235/45 tires, which I saw were 25.3 inches in diameter, or 2/10" larger than the OE. That sounded pretty close, but when I tried them on and lowered the car to the ground, the spring perch was now directly resting on the tire! So front tire diameter is a critical dimension for clearing the strut.
My new tires are 225/45 with a diameter of 25.0" on 7.5" wide wheels and an ET of 45, which worked out just fine. There was a noticeable gain in the space below the spring perch and the - 1/10" difference in dia is probably not enough to throw off any of the functions that rely on rotational speed of the wheels (e.g. ABS, cruise control, etc.)
Finally, I calculated that the OEM width of 8.0" with ET 50 locates the tire relative to the wheel opening pretty nearly the same as my new combination of 7.5" and ET45. Measuring from the hub outward, my OE wheels' outer edge was 51.6 mm and on my new set up is 50.25. Visually, this 1+ mm difference is barely noticeable, the new setup fills the wheel wells just fine.
I'm going four-square with the new tires/wheels, so I will be a little further off in the back. There the OEM outer edge measured from the hub is 53.95, so I'll be about 3.75 mm narrower on each side. I could add spacers for cosmetic purposes but probably won't find it noticeable enough to bother. And the diameter of the 255/35 OEM rears is 25.03" so they're almost exactly the same as the new tires.
The wheels I was putting on came with 235/45 tires, which I saw were 25.3 inches in diameter, or 2/10" larger than the OE. That sounded pretty close, but when I tried them on and lowered the car to the ground, the spring perch was now directly resting on the tire! So front tire diameter is a critical dimension for clearing the strut.
My new tires are 225/45 with a diameter of 25.0" on 7.5" wide wheels and an ET of 45, which worked out just fine. There was a noticeable gain in the space below the spring perch and the - 1/10" difference in dia is probably not enough to throw off any of the functions that rely on rotational speed of the wheels (e.g. ABS, cruise control, etc.)
Finally, I calculated that the OEM width of 8.0" with ET 50 locates the tire relative to the wheel opening pretty nearly the same as my new combination of 7.5" and ET45. Measuring from the hub outward, my OE wheels' outer edge was 51.6 mm and on my new set up is 50.25. Visually, this 1+ mm difference is barely noticeable, the new setup fills the wheel wells just fine.
I'm going four-square with the new tires/wheels, so I will be a little further off in the back. There the OEM outer edge measured from the hub is 53.95, so I'll be about 3.75 mm narrower on each side. I could add spacers for cosmetic purposes but probably won't find it noticeable enough to bother. And the diameter of the 255/35 OEM rears is 25.03" so they're almost exactly the same as the new tires.
Last edited by Tom in Austin; 06-08-2019 at 06:13 PM.
