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Tire size calculator  Rating:  Rating
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 Posted: 07-14-2005 09:13 pm
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OverdriveGear
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Joined: 06-01-2005
Location: Gainesville, Georgia USA
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I thought I'd share this calculator with ya'll. It's pretty cool and saves a good deal of time doing arithmatic. Also, you don't have to "guess" how much your speedo will be off by switching tires. Give it a try:

http://www.miata.net/garage/tirecalcold.html
(the program doesn't like firefox sometimes, so try IE)

With that being said, I was wondering if anyone had all the specs and numbers on the stock JH wheel. I was wanting to do a panasport or minilite wheel upgrade and need to know what exactly to look for. Thank you.

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 Posted: 07-14-2005 11:48 pm
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Mark Rosenbaum
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Joined: 03-12-2005
Location: Kingman, Arizona USA
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The shop manual describes the stock wheel as being cast aluminum with a flat hump and a safety ledge.

Numerically the wheel is 5.5J-13" with an offset of 0.75" giving a backspace of  3.5" (to make this absolutely clear, the car side of the wheel is noticeably deeper than the street side).  Note that the backspace measurement follows the British convention which IIRC is measured at the tire bead mounting surface, not the US convention which uses the innermost surface of the wheel.

The stock wheel mounts via four 7/16" diameter studs on 4.00" centers, not 100mm which won't fit.  You'll probably want your wheels machined for conventional flanged mag wheel lug nuts rather than the stock lug nuts, which are unique to the JH.  Installation torque is 51 ft-lb dry regardless of lug nut type -- and since replacement studs are $5-$6 EACH you will not want to exceed this limit.

The center hole is 2+3/8" diameter but if necessary you should be able to go to 2" or even 1+3/4" before the wheel would interfere with the front hub.  If you're planning on using aftermarket hub caps, make sure they, too, will fit over the hubs.

The inner diameter of the rim is about 12.5" but this tapers considerably so you'd need to check each individual wheel style on the car to see if there were any clearance issues.

As an alternative, you might wish to consider having the stock wheels abrasive blasted and painted or polished.  You can get some very stunning looking wheels that way, at a much lower cost than for new ones.  Of course they're still 13" wheels....

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