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2014 Jeep Summit
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987 Posts
Discussion Starter · #1 ·
Hey Guys,

I just bought some trailhawk 18" rims to use as winter rims, I found a great deal on 255/55/18 blizzak tires will these work on a 14 summit? I searched with no help.

Thanks Guys,
 

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Grand Cherokee TH
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87 Posts
Tire rack says stock is 265/50r20 which would be 30.43” diameter. 255/55r18 would be 29.04”, so too small.
 

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Grand Cherokee
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4,399 Posts
For a 265/50R20 Tire

Cross Section is the 265 number, how wide the tire is (at its widest point, not the tread) in mm.
Profile Ratio is the 50 number, how tall the side wall is as a percentage of the width.
R = Radial
Rim Diameter is the 20 number, in inches.

Yes, tire sizes are done in a mix of metric and standard.

To find the overall diameter, you don't even need an app, just a calculator.

Convert the Cross Section to inches, divide it by 25.4 mm/inch

265mm / 25.4mm/inch = 10.433"

To find out the diameter of the tire, simply add the height of the sidewall x 2 to the rim diameter. You count the sidewall twice, since its on each side of the rim before getting to outside.

How tall is the sidewall, remember the profile number tells you, simply convert the percentage to a decimal (divide by 100) and multiple it by the width, and then by 2.

10.433" X .50 X 2 = 10.433" is the height of 2 sidewalls

Add the height of two sidewalls to the rim diameter, 10.433" + 20" = 30.433" in diameter.

Remember, these dimensions are Nominal, like 2x4 lumber that is really 1.5"x3.5". The actual dimensions will vary slightly, so you can have two different brands of tires with the same size, but their actual dimensions would be slightly different. Not enough to cause any real problems, unless you're talking oversized tires barely fitting within a wheel well, there you could have one brand/model of the same size barely fit, while another brand/model tire would rub. Also, if you have 4WD / AWD, you're tires have to be the same exact diameter. Just the difference in a new tire and tire down to tread bars, will cause differentials to turn more than they were intended, and cause excessive wear on your driveline.

Most tires will have a spec for how many revolutions per a mile, the rack will list it if the manufacturer publishes. That will truly tell you if two tires will spin at the same rpm per a speed, if both tires will turn 833 time per mile, they have to be the same exact diameter.
 
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