Did a search, couldn't find the answer I am looking for.

Install tires with bigger overall diameter (compare to OEM) would throw the speedometer reading off... but is it going to overstate or understate the speed reading??

btw, Mod. if this is not an appropriate forum to post, move as needed.

 

It will understate your speed.

 

Bigger diameter tires (i.e., axle rotates slower for a given speed), speedo reads slower.

 

I think of it this way.

With bigger tires, the car goes further with each axle revolution, so the car doesn't "know" it is going as fast as it actually is (because it assumes the tires are smaller). So the speedo reads low.

 

195/55-15 is the stock size.

Let's use some real examples to help out.

195/50-15, 205/50-15 - both smaller in diameter. More rotations.
195/60-15 - larger in diameter, less rotations.

Plug these numbers into the Tire Size Calculator on Miata.net and you get:



Temporary replacement link: (WTDR down)
Copy and paste in browser, stupid yahoo.
http://www.geocities.com/cnica...e.gif

This shows the percentage difference. A smaller diameter tire will read 'higher' on the speedmeter, which is given in percent on the chart above.

So... on my 195/50-15's, at an indicated 100mph, I am reading 3.3mph faster than I am going, PLUS whatever speedometer error there is inherent in the system. (Since there is no accurate calculation of this error, let's just say it's negligable for sake of discussion...) so you are actually going 96.7mph.

The opposite is true with a larger diameter tire. These will read slower than you are really going. So going 100mph with 195/60-15's, an indicated 100mph is really 103.3mph.

It's pretty minor, when you are talking about tires within 3.5% of each other. Just remember, it is PERCENT, so at 15 mph you aren't really going +/- 3.3 mph, it is much smaller than that.

Hope this helps.

 

Nice! always wondered about that.

 

Awesome bump, almost shy of a full ****ing decade.