I used to know , or partially knew, what is the deal with the whole 255 r40 numbers etc, ?

Is it a measurement in Millimeters or something. and is that just a ratio of tread width to tire height from the rim to the edge?

sorry for being a such a noob, but i need to understand all this stuff in preparation for some slammin and tuckin.

 

As an example: 245/50r16
the first number is the measurement (in millimeters) of the width of the tread.

The second number after the slash is the height of the sidewall as a percentage of the width. For the above example, 50 means that the sidewall height is 50% of 245mm, so it's 122.5mm high.

The r means that it's a radial tire. This means that the steal and polyester plys of the tire run in radially around the circumference of the tire. Years ago, tire plys ran in the opposite way.

The last number is for the size of the rim (in inches) that the tire fits.

There's nothing wrong with asking a question if you don't know... it's better than guessing. but for basic facts like this you probably would have been best off searching for it in a reference guide (online or otherwise).

 

and for those who want to figure out what tire combinations do what...

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