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      04-24-2009, 07:21 AM   #1
MrHarris
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Odometer reading

Anyone know if changing the tires to a larger or smaller diameter affects the rate of the odometer? If so, is there a way to re-calibrate it to make it work with a new size tire?
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      04-24-2009, 04:44 PM   #2
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bump, anyone know?
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      04-24-2009, 05:45 PM   #3
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My logic says that odo reading will be different if the diameter is different (as compared to the exact stock diameter) but I don't know i there is a way to recalibrate the odo based on a different diameter.
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      04-24-2009, 09:52 PM   #4
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There are companies that do that... and it's something I'd recommend so your ABS, etc. isn't all out of whack.
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      04-25-2009, 12:21 AM   #5
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Compared to stock tire diameter.....

Larger diameter will cause the speedometer to read LOWER than actual.

Small diameter will cause the speedometer to read HIGHER than actual.

Thus, with a larger diameter tire, your odo will record less mileage than actual

How you can easily figure out what the "error" is to figure the stock diameter and then figure the new tire diameter. Find the % difference and then apply that to your speedometer reading. For example, on my truck, I went up in diameter. Using the sizes available online for stock and new tires, I was able to figure about a 10% increae in diameter. So, when my speedo reads 64, I am doing about 70 (70.4mph). Make sense?

I am not sure how the M3 picks up the signal for speed (and thus the odo), but in my truck is in the tranny. So, if you were to re-gear the diff, you would have to calculate for that too. I ended up re-gearing my truck and "underdrove" it about 20%. So now, when I read 77, I am only doing 70.
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      04-25-2009, 01:05 AM   #6
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yea it does affect the reading and the post above this explains it clearly ! +1 good explanation .
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      04-25-2009, 01:31 AM   #7
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The odometer will read incorrectly if the speedometer reads incorrectly.
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      04-29-2009, 11:02 AM   #8
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Changing the wheel diameter will affect your speedo reading but should not affect your ABS. However, your speedo is probably not spot on anyway. I believe that the factory is allowed up to 7% error. A speedo that is off will mean the odometer is off as well (as well as gas milage measured from distance).

Check your true speed with a GPS. I have checked many of my personal cars and found that they all read faster than actual speed. I once changed my wheel size to one that corrected my speedo error.
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      04-29-2009, 12:49 PM   #9
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1) This is a calculator for what you're up to:
http://www.miata.net/garage/tirecalc.html

2) Remember this only relevant on the front tyres.
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      03-28-2011, 02:54 PM   #10
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Quote:
Originally Posted by Ken37.73 View Post
1) This is a calculator for what you're up to:
http://www.miata.net/garage/tirecalc.html

2) Remember this only relevant on the front tyres.
Sorry to revive this thread from the dead...

Are you saying the speed is measured from the front wheels?
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