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Old 03-01-2010, 08:40 AM   #1
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Got my math right?

Just trying to see if I have my math right... I just bought a set of Honda VX wheels and was going to buy tires for them. Since the 165/70-13's seem to be fairly limited, I was thinking about going with the Kumho Solus KR21's in a 155/80-13, but the plus seems to be the 155/80-13's are about a close to stock size as I could get anyway.

So for the math...

STOCK SIZE (1995 Civic DX sedan)

175/70-13 would leave a 175*.70=122.5mm sidewall. Wheel diameter is 13in or 330.2mm. Total tire diameter would be 122.5*2+330.2=575.2mm.

NEW TIRE SIZE (on VX wheels)

155/80-13 would leave a 155*.80=124.0mm sidewall. Total tire diameter would be 124.0*2+330.2=578.2mm.

That is a difference of 578.2/575.2-1=0.52%

So at a speedometer reading of 70mph my car will actually be going 70*1.0052=70.37mph.

Does that all look correct? Just trying to make sure I actually know how fast I am going and how far off my MPG calculations might be.
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Old 03-01-2010, 03:22 PM   #2
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http://www.miata.net/garage/tirecalcold.html
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Old 03-02-2010, 07:03 AM   #3
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So yes....
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Old 03-02-2010, 02:07 PM   #4
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Depending on how accurate you want your speedometer, if you went up a couple sizes to something like a 185/80/13 you would probably be able to get a litte better mileage because of the taller gearing, but you'd need to verify the larger tires were not going to create any rubbing issues.
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Old 03-02-2010, 04:08 PM   #5
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Well then speedo and odometer would be off and I would have to do more calculations on my fuel economy to get "actual fuel economy"... lol I am too lazy for all that. Will probably just go with the 155/80-13, especially since tires are almost half as much.
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