Tire size, impact on the tach? - Fuelly Forums

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Old 12-27-2007, 04:47 AM   #1
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Tire size, impact on the tach?

So I have a set of 155-80-13 tires on my VX, and the corect size 165-70-13.

I'm wondering if this got any effect on the mph and the rmp display on the tachometer?
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Old 12-27-2007, 05:07 AM   #2
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Tach measures engine speed, so that will remain unchanged. Speedometer speed, yeah, that is a variable.

But manufactures do not always have accurate speedometers. Easiest thing to do is to check your actual ground speed with a GPS unit. We did a tire change on the Focus to a taller tire with the same width making the circumference larger, and that made the speedometer spot on. One of my other cars reads about 2MPH optimistic.
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Old 12-27-2007, 11:19 AM   #3
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they're 3% larger so the speedo will read 3% faster (this is relative to the old tires NOT absolute (correct) speed) that means that at 100 mph it'll read 3 mph fast and at lower speeds it'll be proportionally less.

At the same time though, that's within the realm of bald vs new tires
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Old 12-27-2007, 04:43 PM   #4
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Quote:
Originally Posted by kamesama980 View Post
they're 3% larger so the speedo will read 3% faster (this is relative to the old tires NOT absolute (correct) speed) that means that at 100 mph it'll read 3 mph fast and at lower speeds it'll be proportionally less.

At the same time though, that's within the realm of bald vs new tires
hahaha so I'm going slower than 55mph, thats maybe why I'm getting so great FE.

thx for the info, and yes I was talking about the speedo and not the tach... thx
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Old 12-27-2007, 05:58 PM   #5
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Originally Posted by BIBI View Post
hahaha so I'm going slower than 55mph, thats maybe why I'm getting so great FE.

thx for the info, and yes I was talking about the speedo and not the tach... thx
Um yea...53.5 mph...big difference.
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Old 12-27-2007, 06:13 PM   #6
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BIBI -

Yup, a GPS is the way to go. Here are my results :

New shoes (aka tires)
http://www.gassavers.org/showthread.php?t=3699

Optionally, you can use this calculator :

Miata.net Tire Size Calculator
http://www.miata.net/garage/tirecalc.html

Or this one :

the wheel & tyre bible
http://www.carbibles.com/tyre_bible_pg2.html

CarloSW2
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