Re: Re: Speedometer off?--spider tires/wheels

[John_Torello@domain.elided]

> I want to put 15" rims on my '85 Spider Graduate.  my question is what
> will > this do to the accuracy of my speedometer.  does anyone know the proper
> 15" > tires that will match height with the 14" stock tires to prevent any
> inaccuracies.  do i need to lower the suspension stop to prevent
> bottoming > the car on the tire?

What you are talking about here is referred to as "plus" sizing. Tire rack has a
good description on their website. The tire calculator programs allow you to
determine (amongst other things) what the overall height of the wheel and tire
combo is. If your 14" wheels with 60 series tires equals a height of, let's say,
24" then a 15" wheel with the same 60 series tire will certainly be taller than
24".(I'm simplifying things here by eliminating the tire width component). That
being said, your speedometer, "bottom out" factor in the wheel well,as well as
performance and comfort will vary. The general idea (the plus concept)is when
you go to a larger wheel, you decrease the aspect ratio to maintain the same (or
very nearly the same) overall height of the wheel/ tire combo. By doing this you
will maintain the same rotation principles as not to affect the speedo. However,
the downside (or upside depending from where you sit) is that the lower aspect
ratio tire is generally going to flex less, providing a "rougher"
ride----although you will generally have better cornering performance due to the
lack of sidewall flex.
Anyway , the folks at tire rack (no association of financial interest blah blah
blah)are usually very helpful with this stuff and can help you with a good tire
that meets your needs.

-JT
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