Originally posted by Troll Pete:
Chris,

You are right and wrong, as are Mike and I. But Mike and I are more correct than you are. While width does effect sideall height, the 2nd number (aspect ratio) effects the sidewall height twice more than the width.

225 x 0.45 = 101.25
215 x 0.45 = 96.75
4.5mm difference

225 x 0.45 = 101.25
225 x 0.40 = 90.00
11.25mm difference

Therefore the common belief and used practice is to increase or decrease the 2nd number (aspect ration) to determine sidewall size.

In addition, it's smarter to change sidewall size.
Why? Because you do not sacrifice width for height. By doing so you get less rim protection on your wheels, increasing the chances of damaging/bending a wheel - especially "curb rash".



The common practice is to increase wheel size while maintaining as close to the original diameter as stock unless one is willing to have the speedometer recalibrated.

example:

stock SVT
Wheel: 16"x6.5"
Tire: 215/50-R16
Width: 215mm (8.464567")
Sidewall: 4.232283" (50% of width 215mm)
Diameter: 24.46457" (sidewall x 2 + 16" wheel)

+1 (upgrade from 16" to 17")
Wheel: 17"x7"
Tire: 215/45-R17
Width: 215mm (8.464567")
Sidewall: 3.809055" (45% of width 215mm)
Diameter: 24.61811" (sidewall x 2 + 17" wheel)

+2 (upgrade from 16" to 18")
Wheel: 18"x7"
Tire: 215/40-R18
Width: 215mm (8.464567")
Sidewall: 3.385827" (40% of width 215mm)
Diameter: 24.77165" (sidewall x 2 + 18" wheel)

On the same car all of the above combos will look pretty much exactly the same when it comes to wheel gap weather the car is lowered or not. The only difference will be the decrease in sidewall/wheel size, the diameter will remain basically the same. With that you get improved performance/handling but less comfort as the smaller sidewall will create a harsher ride. Now if you don't mind a change in speedometer reading then people will increase the sidewall persentage to help close the wheel well gap. However if you venture off the +1, +2, +3 rule of thumb you will change the diameter of the tire and change the output of your speedometer.

If you can handle more width you can also adjust tire size to near the same diameter by playing with larger width tires and different aspect ratios. A safe bump in this direction has been proven to be the 225mm tires in all sizes.

example:
tire: 225/50-R16
width: 225mm (8.858268")
sidewall: 4.429134" (50% of width 225mm)
diameter: 24.85827" (sidewall x 2 + 16" wheel)

difference between 215/50-16 & 225/50-16
width: 0.393701"
diameter: 0.393698"

A little over 3/8" in diameter and not enough to make a big change in speedometer reading so a safe upgrade in most cases. Note: in the above example a bump to a 235mm tire from 215mm would need a change in aspect ratio to 45% and the difference in overall diameter would be around 1/8". Nothing to worry about IF you can handle the width.

same goes with the 18's 215/40-18 & 225/40-18
tire: 225/40-R18
width: 225mm (8.858268")
sidewall: 3.543307" (40% of width 225mm)
diameter: 25.08661" (sidewall x 2 + 18" wheel)

difference between 215/40-18 & 225/40-18
width: 0.393701"
diameter: 0.314964"

less then 3/8" in diameter...again nothing to worry about.

It's all very simple...just do the math.

1st convert "width" mm to "inches" (215mm = 8.464567", 225mm = 8.858268", 235mm = 9.251969")

2nd multiply "width" x "aspect ratio" (50 =.50, 45 =.45, 40 =.40)
sum = "sidewall" (1/2 overall)

3rd multiply "sidewall" x 2 + "wheel diameter" (16", 17", 18")
sum = "tire diameter"



This will get you in the ballpark 99.9% of the time. If you don't care about all this stuff then put on whatever you want to get the look you like. Just be prepaired to get your speedometer recalibrated or face a speeding ticket for not knowing how fast you are going

Scott 2000 Contour SVT #1464 Mustang Dyno: 171.6hp/145.3lb Dynojet Dyno: 171.1hp/148.9lb 1989 20th Anniversary Turbo T/A "Indy 500 Pace Car" #1376 of 1550 All Original, 46k with a few mods 2002 F150 SuperCrew