305/55/20 not 33s??? - Chevy and GMC Duramax Diesel Forum
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post #1 of 11 (permalink) Old 12-10-2011, 05:06 PM Thread Starter
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305/55/20 not 33s???

I just measured my wheel and it measures 32. They terra grapplers.... I thought 305/55/20 was a 33 inch tire? Can anyone confirm? And tell me what is a 33 inch tire that wil fit my 20x9 wheel
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post #2 of 11 (permalink) Old 12-10-2011, 05:18 PM
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yes they do it all the time

LLY dsp5 ad165
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post #3 of 11 (permalink) Old 12-10-2011, 05:25 PM
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305 is the width in mm
50 is the % of that mm on each side (sidewall height)
So 50% of 305 mm is 152.5 mm on each side which is 6" each side (25.4mm=1")
so 6"x2=12" add in the rim height (20") an bam 32"
32.007874" to be exact.

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post #4 of 11 (permalink) Old 12-10-2011, 05:26 PM
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Or just scroll down on this website to the tire height calculator

competitiondiesel

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post #5 of 11 (permalink) Old 12-10-2011, 07:19 PM
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Using Macky's formula and throwing in 55 for the sidewall ratio, you come up up with 33.20866. Using the website Mack posted, it calculates to 33.21. Are you measuring the tire loaded on your truck? Would it make a difference measuring a tire & rim combo loaded on the vehicle versus off the vehicle laying flat on the ground?

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post #6 of 11 (permalink) Old 12-10-2011, 09:46 PM Thread Starter
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I measured them on the truck.
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post #7 of 11 (permalink) Old 12-11-2011, 10:02 AM
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Quote:
Originally Posted by mackyoudip View Post
Or just scroll down on this website to the tire height calculator

competitiondiesel

Here's another website that you can compare the difference between your present or stock tires and another size and how they will affect your speedo.

Miata Tire Calculator

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post #8 of 11 (permalink) Old 12-11-2011, 10:32 AM
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Just realized that this has a 55% sidewall not 50%...
Oops!

Mack: 2014 GMC Sierra gas 5.3
Previous owner of many duramax's fast and slow.
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post #9 of 11 (permalink) Old 12-11-2011, 10:37 AM
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Quote:
Originally Posted by Landito86 View Post
I measured them on the truck.
Weight of the truck makes them shorter.

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post #10 of 11 (permalink) Old 12-11-2011, 12:33 PM
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the best way to measure the acutal size is to measure horizontally not vertically. the weight of the truck will make the tire squish and make it shorter.

My cooper zeon 305/55/r20's measure about 32 7/8"

The height of the tire will change slightly with the width of the rim as well. and 8" wide rim will make the tire slightly "taller" than a 10" one.

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Last edited by melon16; 12-11-2011 at 12:36 PM.
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