Rear ratio calc with different tires - verify my numbers?

[Breadfan]

For the track I got a set of 255/50/16 Mickey Thompson DR's with 16" CVPI steel rims. Considering these are much shorter than stock I knew they would change my effective rear-gear ratio. I want to get a second tune to account for that, so I calculated my new rear ratio with the MT's...

The stock tire diameter for the KDWS is 28.6"

The stock diameter for the MT DR's I have is 26"

I used this equation:

(Orig Diamter * Gear ratio) / New Diamter = effective gear ratio

So... (28.6*4.10)/26 = 4.51

Sound good? So that's pretty close to 4.56, which probably makes things easier, can I just get a second tune with a 4.56 transmission shift schedule/speedo adjustment then.

Sounds close enough to me!

[merc6]

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

When I use the steelies(like now sigh) I used this in the beginning and changed tire revs per mile in the Xcal2.

[juno]

That's close enough. The new tires are 10% smaller, so it should be about 10% increase.

[Breadfan]

Thank you! If I can get my tune I hope to get to the track for a few more runs this season, I know my car has more in it than my sig says. :)

[pantheroc]

I guess that explains why I was 0.5 seconds faster with the tire store's mistake of putting front rubber all around.