tyre calculation

[geckos]

Most of you's may already know .........
...........how to do the calculation from metric to imperial on tyres.

Say if its 235/85 r16 (the size that came standard on my 04 def).
235 width in mm. 85 is the profile of the sidewall in % and 16 is the rim dia.OD is 809mm

235mm devide by 25mm = 9.4 Thats 9.4 inches wide.
To get the height it is...
235 x 85% = 199.75 (Now you have to times this by 2 cause there is sidewalls). 199.75 x 2 = 399.5mm
399.5mm devide by 25mm = 15.98inches. (of both sidewalls)
16inches (rim) plus 15.98inches (2 sidewalls of tyre) = 31.98inches.
OD is 799.5mm Thats 9.5mm difference. Maybe the tread depth?

285/75 r16 285
285mm devide by 25mm = 11.4inches
285 x 75% = 213.75mm x 2 = 427.5
427.5 devide by 25mm = 17.1
16inches (rims) plus 17.1inches (2 sidewalls of tyres) = 33.1inches

Some simex tyres have a larger diameter than what is printed (good in ways).
Hope this helps
Geck

[uninformed]

there is 25.4 mm in one inch:p

alot of the more hardcore tyres dont actually measure what their stated size is.

anoth thing that will play a roll in the mounted size of the tyre is the rim width.

Serg

[LandyAndy]

There is an excellent tyre conversion programme available free on the net.Gives speedo correction too.
Unfortunately I lost the addy when my old computer carked it.
Andrew

[Tombie]

Most of you's may already know .........
...........how to do the calculation from metric to imperial on tyres.

Say if its 235/85 r16 (the size that came standard on my 04 def).
235 width in mm. 85 is the profile of the sidewall in % and 16 is the rim dia.OD is 809mm

235mm devide by 25mm = 9.4 Thats 9.4 inches wide. 235/25.4 = 9.2519"
To get the height it is...
235 x 85% = 199.75 (Now you have to times this by 2 cause there is sidewalls). 199.75 x 2 = 399.5mm 235x1.7 = 399.5mm / 25.4 = 15.7283"
399.5mm devide by 25mm = 15.98inches. (of both sidewalls)
16inches (rim) plus 15.98inches (2 sidewalls of tyre) = 31.98inches. 16+15.7283 = 31.7283" * 25.4 = 805.8988mm - Difference < 3.2mm
OD is 799.5mm Thats 9.5mm difference. Maybe the tread depth?

285/75 r16 285
285mm devide by 25mm = 11.4inches
285 x 75% = 213.75mm x 2 = 427.5
427.5 devide by 25mm = 17.1 427.5/25.4 = 16.8307"
16inches (rims) plus 17.1inches (2 sidewalls of tyres) = 33.1inches 32.8307"

Some simex tyres have a larger diameter than what is printed (good in ways).
Hope this helps
Geck

Worth noting, tyre sizes are always referred to as NOMINAL sizes.

[geckos]

Worth noting, tyre sizes are always referred to as NOMINAL sizes.
Tombie can you explain nominal? im not to full bottle.

[Tombie]

Tombie can you explain nominal? im not to full bottle.

This is the best definition I could find...

Real versus nominal value in engineering - a value that is used as the name for an actual value which is close but not exactly the same.

[2_door]

There is an excellent tyre conversion programme available free on the net.Gives speedo correction too.
Unfortunately I lost the addy when my old computer carked it.
Andrew

I use this one

Tire Size Calculator - tire & wheel plus sizing (http://www.1010tires.com/TireSizeCalculator.asp?action=submit)

[Turtle61]

Here is another one.
This one gives the speedo correction as well as a comparison to 'stock' size.

Tire size calculator (http://www.miata.net/garage/tirecalc.html)

[geckos]

this should all help when people come to getting different sizes

[rick130]

As has been said, they don't always (in fact rarely) measure what their nominal measurement states or calculate out to be.

The most accurate way to measure the diameter of a tyre is to run a tape measure around the OD at inflation pressure and divide by pi. This will give you the unloaded height.

[Turtle61]

This may be true - no one would measure a tyre to see if it's out or not.

However going by defined size ie NOMINAL tyre size, either specified on place card or vehicle specification, and size of tyres fitted to the vehicle - as stated on tyre's sidewall - would give the best method of comparing these two sizes as actual measurement would not be accurate and introduce too many other variables due to construction and tread depth and manufacturing variation of various tyre models and therefore ACTUAL measurement is irrelevant in this argument.

To me, if the specifications say 215/65R16 and the permitted oversize is 15mm extra, then the next size within the limit is 215/70R16. End of story.