275 vs 285

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Apr 9, 2005 | 02:20 PM

MY 95

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From: I need a 4 door C6 Z06

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St. Jude Donor '05-'06-'07-'08

275 vs 285

will replacing the 285's with 275's affect my speedo???

Apr 9, 2005 | 02:39 PM

aboatguy

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From: Slidell Louisiana

If you keep the same aspect ratio the answer is Yes. IE a 285 40 17 has a larger diameter than a 275 40 17. I think/"Believe that a 275 45 17 has a larger diameter than a 285 40 17, so you can find a tire that maintains a near stock diameter. On the other hand I don't think tire size is going to affect the margin of error as much as a gear change and so the error should not be enough to get you thrown in jail.
On my 95 I can change the tire size in the ECM to maintain speedometer calibration.

Last edited by aboatguy; Apr 9, 2005 at 02:43 PM.

Apr 9, 2005 | 02:53 PM

Hotred94

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From: Syracuse Utah

The change will be minor.

Apr 9, 2005 | 03:16 PM

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From: Phila., PA burbs

Very minor difference. After the change, when you speedo reads 60 mph, you'll actually be traveling 60.7 mph.

http://www.miata.net/garage/tirecalc.html

Apr 9, 2005 | 03:18 PM

CentralCoaster

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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed

It'll affect it less than 1%. It would probably vary more between brands than between those sizes.

You can check tire rack, they show tire diameters for various sizes of each brand. If the diameter goes up 10%, the speedo goes down 10%

Apr 9, 2005 | 04:26 PM

CFI-EFI

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From: The Top of Utah

Since you didn't give the full tire sizes, here is how it works. The 275 or 285 is the "section" width. That is the widest part of the tire at the sidewall below (above?) the tread, in millimeters. The 40, 45, or whatever, is the "aspect" ratio. That is the ratio of the section width to the height of the tire from the bead to the tread. Therefore, a 275/40 tire is (275 x .40 =) 110 mm from the tread to the bead. Two bead heights, plus the rim diameter, equals the diameter of the tire. 110 mm/25.4 = 4.330 inches. 4.330 + 4.330 + 17 = 25.66 inches. Tire sizing is an inexact science. Tires from a single manufacturer can vary quite a bit, and even more so among different brands, but this is a good "guideline".

RACE ON!!!

Apr 9, 2005 | 05:08 PM

LT-4 CE

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From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH

Quote:

Originally Posted by CFI-EFI

Tires from a single manufacturer can vary quite a bit, and even more so among different brands, but this is a good "guideline".

RACE ON!!!

Excllent point. I know from when I was racing that even the same sized tires from the same manufacturer often aren't even the same time. Years ago, we used to ask the speed shop to bring out multiples then measure the circumfrence. It was not uncommon to see a 1" variation on the same sized tire. Now most slick manufactures write the circumfrance on the tire.

To your question, it sounds like it has been answered very well. Set the cruise 1mph less and you'll be safe.

Apr 9, 2005 | 09:48 PM

CentralCoaster

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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed

Quote:

Originally Posted by LT-4 CE

It was not uncommon to see a 1" variation on the same sized tire. Now most slick manufactures write the circumfrance on the tire.

Wow! I wonder though if some or most of that was due to them being on the bottom of the stack, but you'd think with steel belts, the diameter wouldn't change much if you sandwiched it..