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Old 10-29-2009, 11:01am   #1
Toque
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Default How to calculate what size tires to get when getting new rims.

Guys:

I have been seeing a bunch of posts recently with folks asking questions about what sized tires to go with there rims. They are also asking if there Active Handling, and Traction control will have issues.

First off, I would like to say that your Rim size has NOTHING to do with the stagger that GM built into these cars. You do not have to upgrade to a 18/19 combo, or 19/20 combo. You can upgrade to 18/18 or a 19/19 combo with no issues. The TIRE sizes is what maintains the built in stagger from GM. As long as you choose the correct tire size then you will NOT have issues with Traction control or Active Handling.

Check out this great little wheel/tire calculator I have been using for years.
Wheel / tire size calculator / comparer - RIMS-N-TIRES

If you input the original tire you have now, and input the proposed size you want to go with, it will output the differences.

*********************************************************************************************

Here is an example of the Calculator output comparing a stock sized rear Z06 tire to a larger tire size (the size I upgraded to in my rear).

Tires:.................Tire 1..................Tire 2
Tire size..............295/35-18...........305/30-19
Section width.......295 mm (11.6'')....305 mm (12'')
Sidewall..............103 mm (4.1'').......92 mm (3.6'')
Tire diameter.......663 mm (26.1'')......667 mm (26.3'')
Rim sizes.............18x9 to 18x11.......19x9.5 to 19x11.5
Circumference......2082 mm (82'').......2094 mm (82.4'')
Revs per mile........773.....................768.4
Speedometer: Assuming that this is OEM tire and speedometer readings are correct.When speedometer reads 60mph (96.6km/h) actual speed will be 60.4mph (97.2km/h): 0.7% faster.

*********************************************************************************************

Stock C5 Tire Sizes, and Stock C5 Z06 to use in calculator:

Using the original size of your tires (see below). Use the above calculator and do the math. If you up size your tires, you want to make sure you up size the front and rear the same amount (or as close as possible). That way you avoid the computer seeing a mismatch in tire speed between the front and rear tires. Thus eliminating Active handling, and Traction control issues.

Stock C5 (97-04):
Front
Rim size = 17x8.5" rim
Tire Size = 245/45/17
Circumference = 2047mm
Diameter = 652mm
Rear
Rim Size = 18x9.5"
Tire Size = 275/40/18
Circumference = 2126mm
Diameter = 677mm

Stock C5 Z06 (01-04):
Front
Rim size = 17x9.5" rim
Tire Size = 265/40/17
Circumference = 2022mm
Diameter - 644mm
Rear
Rim Size = 18x10.5"
Tire Size = 295/35/18
Circumference = 2082mm
Diameter - 663mm

*********************************************************************************************

Here is an example of what I did when I upgraded to a 19/19 combo.
I have had no issues with traction control, or active handling since I got this combo.


Using the calculator above I calculated up sizing my front and rear tires to a size that kept me very close to my stock tire circumference/diameter, AND made sure that both tires increased/decreased the same amount.

Front
Stock Z06 = 265/40/17 = (Circumference = 2022mm)(Diameter 644mm)
Proposed = 275/30/19 = (Circumference = 2038mm)(Diameter 649mm)

Rear
Stock Z06 = 295/35/18 = (Circumference = 2082mm)(Diameter 663mm)
Proposed = 305/30/19 = (Circumference = 2094mm)(Diameter 667mm)

So with my proposed new tire setup:
Front --> Increased the Circumference by only 16mm (Diameter 5mm)
Rear --> Increased the Circumference by only 12mm (Diameter 4mm)

As you can see above (for my situation), both proposed tires are barely larger than my stock Z06 setup. The most important thing is that they both increased the same amount (ish). Thats the key to keep the traction control, and active handling in check.

*********************************************************************************************

I also recommend that you verify the calculator values by going to the tire manufactures web site to verify the sizes this calculator spits out for the specific tire you plan to purchase. Just to be sure...

If something does not make sense above feel free to ask.

Hope this helps !
Toque
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Last edited by Toque; 10-29-2009 at 04:59pm.
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Old 10-29-2009, 11:15am   #2
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Just to add that every mfg sizes their tires differently so not all 335s are the same width. Also wheel width is measured from tire bead to bead, not the acutal width of the wheel. So an 11" wheel is really closer to 12" from outer edge to outer edge.
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Old 10-29-2009, 12:05pm   #3
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Great info Toque - you've got Rep
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Old 10-29-2009, 04:41pm   #4
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Very informative, thanks!

Liked the vids on your homepage. What kind of speed were you doing in your last video at the Texas Motor Speedway where you were following the pace car?
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Old 10-29-2009, 04:45pm   #5
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Quote:
Originally Posted by Munch View Post
Very informative, thanks!

Liked the vids on your homepage. What kind of speed were you doing in your last video at the Texas Motor Speedway where you were following the pace car?
Your welcome !

Although it does not look it, we got as high as 125-130mph. The track is so big it just does not look that fast on the video...

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Old 10-29-2009, 04:58pm   #6
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Quote:
Originally Posted by Toque View Post
Your welcome !

Although it does not look it, we got as high as 125-130mph. The track is so big it just does not look that fast on the video...

Toque
That's what I was thinking. The big boys make those tracks look small the way they move around there.

must have been a lot of fun.
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Old 10-29-2009, 07:08pm   #7
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Quote:
Originally Posted by Cobra4B View Post
Just to add that every mfg sizes their tires differently so not all 335s are the same width. Also wheel width is measured from tire bead to bead, not the acutal width of the wheel. So an 11" wheel is really closer to 12" from outer edge to outer edge.
Always have to throw logic into the mix, don't ya?
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