Tire size information
I have seen many topics all over the site that deal with tires/rim size and how to tell what wont throw off your speedometer etc. Hopefully the following information will help.
First this is how to read a tire. I will use this example. P195/50R15-H
The P = passenger car
195 = section width in mm (thickest point of tire)
50 = aspect ratio. (50% of 195 means sidewall height is 97.5mm)
R = Radial construction (as opposed to bias ply or bias belted)
15 = Rim diameter in inches
H = Speed rating (H is good to 130mph)
Calculating Overall tire diameter:
cross section width X 0.aspect ratio X 2 / 25.4 + wheel diameter
(the 25.4 converts the mm to inches)
Example: 195/50 R 15
195 X .50 X 2 / 25.4 + 15 = 22.677"
Speedometer and your wheels:
your speedometer is set to read the number of revolutions made by your tire to figure the speed of your vehicle. Thus if you have a larger total diameter of tire your speedometer will think you are going slower than you are, because your tire will travel more distance per revolution.
So if you want to keep speedometer accurate use formula above. (The size tire I was using is the one currently on my Accent, now let's take a look at what came with it.
175/70 R 13 = 22.646 inches so I am within .03 inches total tire diameter the difference is not noticeable and about the same as minor wear on your tread.
Speed Rating:
S = up to 112mph
T = up to 118mph
H = up to 130mph
V = over 130mph
z = over 149 mph
This is the speed at which the tire will basically be unsafe because it will litterally come apart. If a service description is present maximum speed is indicated by service description.
I think this covered most of the posts I have seen on the site. If it's not let me know and I'll see what I can do to get some more info.
PS: aspect ratio is important, I have seen alot of bad information on this. Don't tell people that if they want to keep their speedometer correct that a 205/45 205/50 and a 195/50 are all ok to interchange. (on a 15 inch rim in order those were 22.2637, 23.07, and 22.677 a half inch will equate to only 1/4" ride height barely noticeable to the eye but the half inch will be noticeable by your speedo.
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