http://powerdog.com/tiresize.cgi

 

That's not bad, but here's one that provides that info, plus usefull stuff like additional outboard width, so you can gauge rubbing:

http://gs.tolan-hoechst.com/tirecalc.htm

 

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

http://www.onlineconversion.com/tire...conversion.htm

http://www.csgnetwork.com/tireinfo4calc.html

 

cmon guys crunch the numbers yourself!

here is how overall tire diameter is calculated based on the size you see stamped on the sidewall:

205/55-16

205 is width (contact patch) in millimeters.

55 is the aspect ratio of sidewall:width, ie. 55% of 205mm is the sidewall height = 112.75 mm.

16 is the rim diameter in inches.

to calculate overall tire diameter, all you are doing is adding the rim diameter to 2x sidewall (since you are crossing both sidewalls with diameter).

so for 205/55-16,

205mm x 0.55 = 112.75 mm (sidewall height),

112.75mm x 2 = 225.5 mm (both sidewalls in millimeters)

225.5mm / 25.4 = 8.878 inches (both sidewalls in inches, since there are 25.4mm per inch)

8.878 inches + 16 inches = 24.878 inches overall wheel/tire diameter.

the overall equation is:

{ [ ( width x aspect ratio ) x 2 ] / 25.4 } + rim diameter

unless you have a clear goal by usiing a different overall diameter, i suggest sticking with stock diameter (~25 inches) when going to 17 or 18 inch wheels.

to use a smaller overall diameter means gearing and acceleration will be slightly improved, but load rating will be less. also your odometer will log more miles than youve gone, which is negligible, imo.

to use a larger overall diameter means gearing and acceleration would suffer a bit, but load rating will be more. many do this to get a wider tire.

if you want to stick with stock wheel/tire diameter when going with bigger tires, here are the sizes:

16 inch rim = 225/50-16
17 inch rim = 225/45-17
18 inch rim = 225/40-18

now if you do the math, you'll notice that these sizes are not exactly 24.878 inches in diameter, but close to 25 inches. these are the sizes that are closest to stock size and within the limits of all the rims we would buy for suby's.

another important thing to remember is that not every 225/45-17 tire will be the same diameter because certain manufacturers spec their sizes a bit wider or narrower than others. one tire that comes to mind is the Falken Azenis Sport RS, whose 225/45-17 is wider than most other 225/45-17 tires, which in turn makes it slightly bigger in diameter.

 

theres also the +20/-5 rule. given any tire size, if you add 20mm to width and subtract 5% from aspect ratio (profile), the wider tire will have almost the same diameter as the narrower one. e.g., a 225-45-17 tire will have almost the same diameter as a 245-40-17.

 

That is part of what is called the 'Plus Sizing Concept/Theory'. I am sure everyone can go to TireRack's site for a detailed explination, but the gist of it states that to go to a wider tire on the same size wheel (aka 'Plus Zero') you add 10 to the width, and subtract 5 from the sidewall height. In other words, you would go from a 175/65/14 to a 185/60/14. To go with a larger tire & wheel (aka 'Plus One') you add 10 to the width & subtract 10 from the sidewall height (and mount said tire on a wheel that is 1 inch bigger in diameter). So you would go from a 205/55/16 to a 215/45/17. BUT it is a theory so oftentimes you can add 20 to the section width and subtract 5 (or 10 for a Plus One) from the aspect ratio and end up with a tire that is a better match to the OE's Overall Diameter. Sometimes you add 30 to the width. Lets say you have a 04 Mustang - the OE tires might be 225/55/16, 245/45/17, or 275/40/17. If you had a modded Cobra you might consider 315/35/17s...blah, blah, blah...

Ultimately the best way to figure out a tire's OD, width, etc is to go to them. Most size information is available on their websites.

 

actually, with plus zero, the +10 / -5 rule yields a tire size that has a smaller O.D. than the original size. BUT im sure they suggested this instead of +20 / -5 so that people dont put tires that are too wide for their rim widths. if you do the calculation from 225-45-17 to 235-40-17, you'll notice that the 235-40-17 has a lot smaller O.D. than the 225-45-17, even check the tire calculators. i've noticed that to stay almost exact as the given tire size, you have to do +20 / -5.

EDIT: yeah i just noticed that i repeated what you said... so yeah we are in agreement

 

 

It is a theory/concept because it does not hold trut in every case - not so much a hard & fast rule, but a general guideline.

FYI those are real, factory endorsed examples that I listed - for a 92-95 Civic and IS300/WRX/05 Legacy.

(This post is not for Yin - he/she pretty much knows his/her stuff - but for the listening audience out there in TV Land.)


http://www.tirerack.com/tires/tirete...al/plusone.jsp