Re: Tire size

[Howard R Pletcher <n9ads@domain.elided>]

On Sat, 29 May 1999 15:19:21 -0700 MikeIIDC@domain.elided writes:
>Okay, 
>
>Went out this afternoon to get new tires for the Wagonmaster.  I have
>limited funds right now, so I was looking for something reasonable,
>which is unusual for me.  I found something, finally, but, of course,
>the local yokels at the parts store didn't know a whole lot.  Anyway, I
>have 32x75R10.5's on there now.  Wheel is a little wide-10" I think?  I
>guess, after talking to them, that I had the measurement system on tires
>all wrong.  Can anyone tell me where (on the internet, hopefully) I can
>find a conversion chart for the P235 to what in inches is?  They have a
>book on it, but couldn't find it!  
>
>
Actually you are missing one number to be able to determine the diameter
of the tire.  The complete size should be something like P235-75x15.  The
P means a passenger car tire, the 235 is the tire width at the widest
point in mm, the 75 is the aspect ratio (the percentage that the height
of the tire is of the 235 mm width, and the 15 is the rim diameter in
inches.

To get the approximate diameter, let's first convert the measurments to
inches for us metrically challanged--235 mm / 25.4 mm/in. = 9.25" section
width.

9.25 x .75 = 6.9" section height.

The tire diameter will be the 6.9" section height on the bottom plus the
15" rim plus the 6.9" section height on top or 28.8".

If it happens to be a 70 series tire (or a 60 or a 50, etc.) the diameter
will be less for a given tire width.

BUT YMMV!  The whole tire sizing game seems to be done rather
optimistically, perhaps based on some design sizes that are not the
actual tire size--about like TV sets some years back when you had measure
across the face of a picture tube and onto the sides to to a particular
seam to get the 19" they claimed it was.  At least in the case of the LT
tires that are marked with a supposed diameter, I haven't seen the sizes
as that accurate. My 31x10.50's are really closer to 29"--perhaps they
were 30" when new, but not 31".

The original tires on your WM were probably H78-15's, about the
equivalent of a P225-78 I'd guess, on 5.5" rims.  L78's on 6" rims were
the biggest options offered, but I'm not sure the usage was very high. 
Whatever you do will probably be a substantial step up in size.

Howard Pletcher
Howteron Products Scout Parts

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