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My research -
My first step was to use the tire size calculator at
http://www.wickedbodies.net/Tire-Size-Calculator.htm, since it will let you enter "R" as the aspect ratio when comparing two different size tires.
I compared two tire sizes, 185/R-14 and 185/x-14, plugging different values into "x" till I found the one that matched the circumference dimension the calculator gave me for the "R" tire. 82 was a perfect match, so I know that calculator converts "R" to an aspect ratio of 82 before running its comparison calculations.
Basic size information - tire sizes:
First # in tire size is the section width
Second # is aspect ratio
Third # is the rim diameter
Next I used the tire dimensions provided on the "specs" pages at tirerack.com.
I searched for tires in size 185/R-14 and found several - maybe six?? Not that many but enough to do calculations with.
Now by using the tire size formula in reverse I was able to derive the tires' sizes in conventional terms, from their actual dimensions on the specifications pages. Those tires calculated out to being size 185/80-14. (see notes on the calc process further down)
I do wonder whether possibly those factories are set up only to build tires in current standard dimensions. An 82 aspect ratio is between 80 and 85 so maybe they're making the tires 80 out of convenience when back in the 1980's they actually were built as 82? I have no way of knowing.
Another check would be to test the same reverse-calc process on some conventionally sized tires like 195/75-14. Get their dimensions, calculate the tire size from that, and verify the aspect ratio is 75 (in the case of that particular tire size).
I won't quote the formula (it's been a while) but the guts of it are:
Tire overall radius is
Rim radius (for a 14" rim, half that is 7")
Plus the sidewall height.
Times two for the overall diameter.
Section width is NOT the tread width. Section width is the maximum dimension across the inflated tire sidewall, front side to back side.
Sidewall Height and Aspect Ratio:
Section width x aspect ratio = sidewall height
So if section width is 185 (mm) and aspect ratio is 70 then
185 x .70 = 129.5 mm. sidewall height.
You can add the rim radius to the sidewall height to get the overall tire radius, then multiply x 2 to get the overall tire diameter.
Since the rim is 14", the radius is 7".
7" x 25.4 mm/inch = 177.8 mm rim radius
Add the sidewall height of 129.5 mm + radius of 177.8 = 307.3 mm overall radius
307.3 mm. overall radius x 2 = 614.6 mm overall diameter
614.6 mm. / 25.4 = 24.197" overall diameter
for the 185/70/14 tire.
I just looked up a Yokohama tire, 185/70-14 on TireRack, and it's spec'd at 24.3" overall diameter. Off by .1 inch from the calcs above. Several others also were spec'd at 24.3". Drive it 10K miles and it will probably be a perfect match. Dunno if that's the reason for the difference, or if there's another reason.
Anyway, the difference between 185/R-14 (orig. size) and 185/75-14 (offered snow tires) is at least .72" diameter, so the .1" discrepancy I found above is insignificant in comparison. .1" is far less than the tire will change in its lifetime due to treadwear.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245
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