Miata Mailing List: September 1994, Message #134
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   From: dquara@encore.com (Doug Quara)
Subject: tire sizes (was: Re: finally did it.)
   Date: Wed, 7 Sep 1994 15:56:01 -0400

Carlos Chang, On Sep 7,  1:12pm, writes:
> Subject: finally did it.
> Ok, I finally did it!  I've comfirmed my order for a laguna blue, 
> 94' B-package.

Congrats!  Many pleasurable years and miles to you and your new car!

> Should pick it up this week.  My question is for tire size.  I'm getting rid
> of the original brickstones it comes with.  I plan to replace them with 
> yokohama A509s.  Should I go with the same size, new yok's or take the 
> "almost new" 205/60/14's off my 240Z and put them on the miata?  
> I believe the original sizes are 185/60/14.
> 
> Will there be a noticeable difference in rolling resistance, mileage,
> acceleration and top speed?  I'm mostly concerned about the acceleration.

Approximate tire sizes:

	185/60/14:   22.7402" diameter
	205/60/14:   23.6850"
	205/50/14:   22.0709"

So, the 205/60 compared to the 185/60 changes your final drive ratio
by a factor of .96 (reducing your RPM at a given speed by a factor of .96, 
or said another way, increases your speed at a given RPM by a factor of
1.04).  

There will be a penalty in acceleration, but I don't know if it significant.
The 205/60 is nearly a full inch taller.  There will be higher rolling
resistance from the 20 mm wider section width, assuming the same brand/model; 
switching brands is apples/oranges, and all bets are off for 
mileage, grip, etc. 

I don't have the physics background to tell you if the decreased final
drive ratio will affect top speed or not.  Theoretically, it should
increase it, but it depends on the torque available at the new "threshold"
speed where the RPM hits the aerodynamic wall of air.

You can decide if you want the spend $$ on new tires.  Personally, I'd
wear out the brickstones (taking special care in the wet), then
replace them.  In fact, that is exactly what I'm doing with my '94.  
I work on this every day.  :-)

(Note: tire sizes noted above are approximate and vary from manufacturer
to manufacturer.  Most good tires stores will have tire diameters for
each model/size of there tires.)

Hope this helps,

Doug Q.
dquara@encore.com