Miata Mailing List: December 1997, Message #596
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From: 	"John Freas" <jfreas@gte.net>

Subject: Re: Speedometer difference with new tyres!

Date: 	Fri, 5 Dec 1997 00:31:40 +0000



Previously on Speedometer difference, parin pmody@usr.com said...



>         # i am a proud new owner of '90 miata and have 

>         # yokohoma A509 tyres, i think the dimensions are

>         # 205/50/R15 .. and i wanted to know if any of the 

>         # miatae gurus can tell me by how many miles will 

>         # my speedometer be off?

>      

>         # if not, can someone tell me the dimensions of the 

>         # tyres that come with the stock miata?



Hey, I can tell you both!  Cool, I get to be a guru!



The dimensions of the OEM tyres are:



Sidewall: 111 mm

Radius: 288.8 mm

Diameter: 577.6 mm

Circumference: 1814.58 mm

Revs/km: 551.091



Assuming you are right about your A509's dimensions, then they are:



Sidewall: 102.5 mm

Radius: 293 mm

Diameter: 586 mm

Circumference: 1840.97 mm

Revs/km: 543.191



The difference in revolutions per kilometer is

543.191/551.091=0.9856647994614 (or thereabouts)



Call it 98.6% of the factory tyres.  Therefor, your speedo will be off by

1.4% and it will vary in actual kph according to how fast you are going. 

For example, at 30kph you will be indicating 29.57kph, but at 120kph you

would be indicating 118.28kph.  IOW, the error increases with speed.  In

any case, it isn't significant.



FYI, the OEM tyres are 185/60/R14.  For this and other ways to impress your

friends, try:

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



Fun for all ages.  Maybe they'll call you a guru too!



	-John



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John Freas (jfreas@gte.net)

Red '90 Base-ish #7690

Deer Park, Long Island

Team Laguna Seca

Team PBC

Team Roebuck

Team Voodoo

Team "I have no idea what those dimple things on the windshield frame are

for."

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