Figuring out your ride height after adding 17" or 18" rims
12-29-2005, 03:07 PM
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Figuring out your ride height after adding 17" or 18" rims Sidewall Aspect Ratio
Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.
P225/50R16 91S
The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 ¿ 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall. (www.Tirerack.com)
So, we can infer that for as long as the tire width is the same, that for every 20% of difference in increase or decrease of tire aspect ratio we can figure a 1.8" increase or decrease, or for every 10% (1/2) a .9 inch increase or decrease. But this only for this tire size.
Now, figuring out our ride height.
Standard equipment on a Trans am is 245/50-16.
This size tire section width = 4.82".
If your are going to 17 inch rims, those standard numbers will be 275/40-17. This size tire section height = 4.32".
4.82-4.32= .5
This would equal a .5 inch decrease in the side wall of the 17 inch tire vs. the 16 inch tire. With the one inch increase in rim size results in a .5 inch increase in ride height, but only with those tire section widths and sidewall aspect ratios.
If we kept the section widths the same between the 16 and 17 inch tires, we would get a one inch increase in ride height. If we had decreased the section width of the 17 inch rim tire even more, in addition to the sidewall aspect ratio, we could achieve an equal or lesser ride height.
REMEMBER, the sidewall aspect ratio number is the tire section height, only it is given as a percent of the tire section width which is in millimeters. Thats why we convert to inches by taking mm and dividing by 25.4. Do this for both tires, mutiply by the percent. Take your two answers, subtract, and the difference is your increase or decrease in tire size wall height. Subtract this number from your increase in rim size to figure out the increase or decrease in ride height.
Now let's do this for 17- 18's!! If you are going to 18 inch rims, the standard numbers will be 245/40-18 all around or 275-35-18 for the rear.
With 245/40-18
Tire section height= 3.86
Tire section height of 16's= 4.82
Tire section height of 17's= 4.32
4.32-3.86= .46 decrease is sidewall height. We increased one inch, but we are now .54 inches higher. If we go from 16's to 18's, we are now sitting 1.04 inches higher.
With 275/35-18
Tire section height= 3.75
Tire section height of 16's= 4.82
Tires section height of 17's= 4.32
4.32-3.75= .57 decrease in sidewall height. Therefore, we are now sitting only .43 inches higher in the back. Going from 16's, now only .93 inches higer.
Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.
P225/50R16 91S
The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 ¿ 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall. (www.Tirerack.com)
So, we can infer that for as long as the tire width is the same, that for every 20% of difference in increase or decrease of tire aspect ratio we can figure a 1.8" increase or decrease, or for every 10% (1/2) a .9 inch increase or decrease. But this only for this tire size.
Now, figuring out our ride height.
Standard equipment on a Trans am is 245/50-16.
This size tire section width = 4.82".
If your are going to 17 inch rims, those standard numbers will be 275/40-17. This size tire section height = 4.32".
4.82-4.32= .5
This would equal a .5 inch decrease in the side wall of the 17 inch tire vs. the 16 inch tire. With the one inch increase in rim size results in a .5 inch increase in ride height, but only with those tire section widths and sidewall aspect ratios.
If we kept the section widths the same between the 16 and 17 inch tires, we would get a one inch increase in ride height. If we had decreased the section width of the 17 inch rim tire even more, in addition to the sidewall aspect ratio, we could achieve an equal or lesser ride height.
REMEMBER, the sidewall aspect ratio number is the tire section height, only it is given as a percent of the tire section width which is in millimeters. Thats why we convert to inches by taking mm and dividing by 25.4. Do this for both tires, mutiply by the percent. Take your two answers, subtract, and the difference is your increase or decrease in tire size wall height. Subtract this number from your increase in rim size to figure out the increase or decrease in ride height.
Now let's do this for 17- 18's!! If you are going to 18 inch rims, the standard numbers will be 245/40-18 all around or 275-35-18 for the rear.
With 245/40-18
Tire section height= 3.86
Tire section height of 16's= 4.82
Tire section height of 17's= 4.32
4.32-3.86= .46 decrease is sidewall height. We increased one inch, but we are now .54 inches higher. If we go from 16's to 18's, we are now sitting 1.04 inches higher.
With 275/35-18
Tire section height= 3.75
Tire section height of 16's= 4.82
Tires section height of 17's= 4.32
4.32-3.75= .57 decrease in sidewall height. Therefore, we are now sitting only .43 inches higher in the back. Going from 16's, now only .93 inches higer.
