How to calculate tire sizes...
 

Hey guys .. I've been asking myself this for a while... I'm gonna be getting 16 in rims soon ... I'd like to know what size tires would be best to keep everything proportionate. Does anybody have some way of calculating it ?

Thanks alot people !

Rick

Oh yeah. :)

Tires sizes are like 185/70R13. The 185 is the width of the tire carcass in millimeters. The 70 is the section height - the height of the sidewall, measured as a percentage of the section width. (70% in this case) 13 of course is the rim diameter in inches.

So to get tire diameter, you need to find the sidewall heights and add it to the rim diameter. So it's 70% of 185 (129.5) times two (259), converted to inches (divide by 25.4, 10.19), added to the rim diameter (23.19).

Simple. (I worked this out when I was like 4 or 5... math geek from the start)

So the tires are 23.2" in diameter. You want 16's? 23.2 minus 16 is 7.2". Times 25.4 is 182.88. Divided by 2 is 91.44.

So the sidewall has to be about 91.44mm high. A 40 series tire (divide by .40) makes 228.6 - so a 225/40-16 would be the same height, about. A 45 series tire (divide by .45) is 203.2 - so a 205/45-16 would be the same height, and is really what most people use.

a search would have solved this question... but to give you a start,

lets talk about a 225/50R-15" tire.

it is 225mm wide
for a 15" wheel the "50R" designates the "aspect" RATIO or percentage of height vs width of tires.
so the rubber between the wheel, and the tread would be 112.5mm...

to figure out height (from which you can calculate circumference) you have your aspect ratio times 2, (since there is rubber on the bottom of the wheel, and above the wheel) so back to 225, plus your 15" wheel.
225=9" so the tire would have a height of 24"

using that same logic you should be able to calculate what you need...

The "R" means it's a radial. A "B" means it's a bias-ply, and a "D" means it's a bias-belted.

Last time I saw B- and D- tires in a P-metric size (as opposed to alphanumeric) was in the early 80's when I used to go through the tire section of the Sears catalog. Calculating tire sizes. For fun.

OMG .. thanks guys ! loll .. so .. quick ! loll

So all and all .. I'll go with the 205/45-16 , 45 series tires ... I really wanna keep it all the same ... And 16s I find aren't TOOO big, they leave you with enough rubber and all so .. perfect fit ! :D

THANKS !

I have got a funky Excel spread sheet that does Tyre rolling diameter (given diameter, profile and width ) calculations as well as Gearing (Speed in Gear, RPM in gear) calculations.

If someone wants it and/or wants to host it I will email it to them.

Or you can go to the garage at Miata.net and use their Java version.

Here you go... a link for tire size comparison

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

and another for offset
http://toy4two.home.mindspring.com/offset.html

That should get your answers without a calculator :bigthumb: