rolling diameter for dummies
 

Can someone break down the math for me? I know an inch is ~25mm, But I've been told the width plays a factor also. How much of a role does this play in rolling diameter with a reasonable PSI? Say I move from 16s to 17s and want to keep the rolling diameter ~equal, for obvious speedo issues. Is it as simple as getting a tire with ~12.5mm taller side wall rating? Thanks,

Theoretical rolling diameter can be calculated quite easily as the section (or profile) approximates the percentage ratio of sidewall height to tread width -

1. convert wheel size to mm
2. sidewall height = tread width x section (225*0.4, 265*0.35 for example)
3. RD = wheel size + 2 x sidewall height

MS Excel is an easy tool for charting different options. I say theoretical because manufacturers vary in their construction, tread width and section % are not always in correct proportion.

Cheers, Maurice :cool:

There are lots of factors that will contribute to a given tires rolling radius, not the least of which are the ideosyncasies of each manufacturer. The best thing to do is to go to each individual manufacturers web site for the correct relevant info of the tires you are interested in. Yes, wheel width does affect rolling radius and section width by stretching/compressing the cross section of the tire, a wider rim stretches the tire axially giving a greater section width and at the same time a shorter rolling radius.

At the manufacturers web site look for the following info

• measured rim
• section width
• overall diameter
• static loaded radius
• rolling circumference
• revs/mile

Each gives some info that is useful to know when choosing a particular tire

DOAH! Shows how much I know. All this time I thought that the section number was a mm measurment not a %age of the width... I appreciate the education. Thanks,

How to read a sidewall;
245/45R16
245 is the nominal section width in mm
45 is the aspect ratio = section height/section width
R-radial
16 rim diameter in inches

section height = distace from the bead seat to the outer tread surface of the inflated tire
section width = distance between the outer sidewalls of an inflated tire

here are some examples form some tires i once had on my Carrera;
front nominal size 225/50R16 on an 8x16" rim, 248/36R16 when measured as installed

rear nominal size 245/45R16 on 9x16" rim, 260/34R16 when measured as installed

Yes, the middle # (aspect ratio) IS a percentage of the WIDTH of the tire- in other words, when people refer to 45 "series" tires, or whatever, they are icorrectly implying that a 245/45/16 would be the same as a 315/45/16 & this is WRONG- a 245/45/16 has a RD of 24.68"- the width is 245mm & the profile is 45% OF 245mm- therefore, 110.25mm- so, you convert the 110.25mm to INCHES by dividing 110.25 by 25.4 & get 4.34"- you double this # b/c you have profile on TOP & BOTTOM (dealing w/diamater VS radius) & you get 8.68- add this to 16" & you have 24.68" RD. A 315/45/16 (IF there IS such a tire) would be 141.75mm of profile- MUCH taller profile than a 245/45 & divide by 24.4 & get 5.58"- DBL this & get 11.16" & add to 16" & get a RD of 27.16"....

you need to match the RD whenever going to a larger (or smaller) rim or you change your speedometer AND by extension, gearing... In the 951 world, most people like to go from a 16" to a 17" rim- the standard 944 turbo had 225/50/16 rear tire (24.86" RD) & the Turbo S, a 245/45/16 (24.68" RD) & ususally go to a 255/40/17 (25.03" RD)- this gives a tiny bit more footprint (255mm VS 245mm) AND a slightly taller (yet VERY CLOSE) RD match- it will HURT accel a TINY bit, as it is taller, but, is still well w/in the 2% (+or-) range of error that is considered acceptable...

Try this site. It's a good one for comparing tire of different sizes. http://www.miata.net/garage/tirecalc.html