Sychronizing Speedo w/ bigger wheels/tires - TCCoA Forums
 
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post #1 of 5 (permalink) Old 02-22-2004, 12:19 AM Thread Starter
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Sychronizing Speedo w/ bigger wheels/tires

I am about to put my 255/50/17's on my car. How bad, if any will it throw off the speedo, and how do I correct it? Also I am gonna put my 15's back on next fall to drive thru the winter. How can I find a 15" tire combination that will be the same overall diameter of the 255/50/17's? Thanks, BZ

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post #2 of 5 (permalink) Old 02-22-2004, 12:44 AM
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Quote:
Originally posted by Bradley Blaze
I am about to put my 255/50/17's on my car. How bad, if any will it throw off the speedo, and how do I correct it? Also I am gonna put my 15's back on next fall to drive thru the winter. How can I find a 15" tire combination that will be the same overall diameter of the 255/50/17's? Thanks, BZ
That is 4% over. so if you are going 60mph on the speedo, you are actually doing 62.4

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

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post #3 of 5 (permalink) Old 02-22-2004, 09:23 AM
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The speedometer calibration is done by changing the number of teeth on the driven gear that the speed sensor reads from. Since you're making a fairly small change in diameter, the next step fron what you have for a gear now would be too much of a jump.

The SCCoA site has a speedo gear calculator that's a bit easier to use for an MN-12 car, because it has a few of the variables listed for you already.

http://www.sccoa.com/faq/speedgr.html

As for 15" tires that will match the diameter of the 17" tires, 215-70-15 would be the closest, within 1/4".

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post #4 of 5 (permalink) Old 02-23-2004, 12:48 PM Thread Starter
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ok, how do I find out my rear end ratio, somrhting in the door jamb sticker right?

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post #5 of 5 (permalink) Old 02-23-2004, 01:19 PM
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Quote:
Originally posted by Bradley Blaze
ok, how do I find out my rear end ratio, somrhting in the door jamb sticker right?
yeah, it will be a single digit number under the AX key on the door jamb decal. The reference table is in the tech articles.

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