Re: Tire Sizes and how to calculate

[Rob Palmason <rpalmason@xxxxxxxxx>]

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Related to this topic, here is a summary of the tire conversion math
from Tire Rack.

----------------------

Calculating Tire Diameter
Example...185/60R14 85H or 185/60HR14

The first number is the width of the tire in millimeters, measured
from sidewall to sidewall. To convert to mm to inches, divide by 25.4
In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall
height to tire width. In the example above, the tire is 7.28" wide,
multiply that by the aspect ratio to find the height of the sidewall.
In this case, 185x0.60=111mm or 7.28"x0.60=4.36". (ie: the height of
the sidewall is 60% of the width of the tire).

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and
multiply by 2,(remember that the diameter is made up of 2 sidewalls,
the one above the wheel, and the one below the wheel) and add the
diameter of the wheel to get your answer.

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"
Or for us non-metric types
7.28” x .60 = 4.37 x 2 = 8.74 + 14 = 22.74” overall diameter

Rob
Littleton, Massachusetts




On Wed, Jun 10, 2009 at 12:52 PM, Layne Grissman<64Polara500@xxxxxxxxx> wrote:
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> Wow, Good post.   And I will agree, too many of the "Car and Tire"
> Shops out there depend too much on the computer...
> As for the 3.91's, well, we got the car with the 3.91's in her,
> however it was not original, as the person we bought the car from many
> years ago made some very significant changes to the car.  This one had
> a 413 in it from a '63 Imperial, so we built it up...It was a Max
> Wedge Block casting...We bored it over .030, and added dual fours to
> her, new cam, new Edelbrock Performer RPM heads and Doug's Headers to
> her, so she will be a performer to an extent...., it was originally a
> 318....as well as the 489 case and 3.91 gears, which from what we know
> were originally 3.23's....non sure grip....
>
> This car is not stock and we are not worried about the originality of
> it.  Things would have been easier if we only went with original.  We
> are not tubbing it out, we are not making body changes, and we are
> keeping it as close to stock as the day we got it, if that makes any
> sense.   We're just trying to work with what we have, hence the reason
> I asked the question.
>
> We were thinking about going with 29" tall to try and compensate a
> little but I wanted to be sure and get a little more specific on
> exactly what we needed...
>
> This is why we are trying to figure the proper size.
>
> Hope that all makes sense...
>
> Layne
>
> On Wed, Jun 10, 2009 at 11:32 AM, Herb<zephyr9900@xxxxxxxxxxx> wrote:
>>
>>
>> When your replacing tires on your car you need to get tires the same height
>> as were installed from the factory such as a 27 inch tall tire was the
>> average for our B body's.  Rim diameter dosen't matter or the width of the
>> tire other than rubbing on the inner or outer fenders.  If the 3:91s were
>> stock and you change your tires to a taller tire you will throw your
>> speedometer off and on new cars with a computer a Lot More.  If the 3:91s
>> were not stock and you installed them you are completely defeating the
>> purpose of installing 3:91s and your wasting all the money you spent on them
>> going with a bigger tire!   You should have thought about RPMs & MPH before
>> the gear change and not in your tire choice!!  If the tire shop your dealing
>> with cant help you with a 27 inch tall tire in any width or rim size they
>> are idiots that you need to run away from them ASAP!!  Their are several out
>> their that haven't got a clue as to how to properly fit a tire to a car
>> other than what the book/computer says.  Was looking for tires for my
>> Durango last week and was told by a tire shop that I had to buy new rims &
>> tires for it because no one makes tires for that size rim on Durango's
>> anymore.  Stunned I asked if they don't make 285/50 R20s anymore?  They said
>> yes but not for a Durango!  They need to close their doors and go away in
>> embarrassment.  Now if you are trying to get better gas mileage the best
>> thing to do is change your 3:91s to something more streetable such as a 3:23
>> or 2:73s but that is not always affordable & don't forget your tranny gear.
>> The other problem other than your spedo is if you go to too tall of a tire
>> to make a difference in offsetting the 3:91s, your going to have to invest
>> in air shocks to keep things from rubbing and remember the taller the tire
>> the narrower it will have to be to keep from rubbing the outer fender well
>> making your Polara look goofy driving down the road.  What area do you live
>> in I have a 3:23 & a 2:73 sure grip that we can work out a trade for your
>> 3:91 or something.  Sorry for such a long winded reply but their is a point
>> that form over function dictates the avenue followed but, it is your car!!
>>
>>         
>>  Herb
>>
>> 1956 Plymouth Belvedere 361
>> 1959 Coronet 326 Poly
>> 1963 Fury 2D/HT 6.1
>> 1963 Sport Fury Convertible 361
>> 1970 Challenger RT 440
>> 1999 Durango SLT 5.9
>> 2006 300-C Heritage 5.7
>> 2008 SRT-8 Magnum 6.1
>> St. Louis, MO.
>>
>> http://1962to1965mopar.ornocar.com/mmo42009.html
>>
>>
>>
>>
>> -------Original Message-------
>>
>> From: Layne Grissman
>> Date: 6/9/2009 10:58:41 PM
>> To: 1962to1965mopars@xxxxxxxxxx
>> Subject: Tire Sizes and how to calculate
>>
>> Does anyone know how to calculate final tire size when trying to keep
>> the RPM's down?  What is the typical RPM range I should try to keep it
>> in and at what speeds?  I was figuring about 2800 RPM at about 60-65
>> mph.....We have a 3.91 Sure Grip rear...Want to get the right tires
>> the first time....  :)  Is there a formula?
>>
>> Also was the typical final gear on the A727 1.00??
>>
>> Thanks all!
>>
>> Layne
>> '64 Polara 500
>> '08 Charger
>> '08 Ram (yeah it's got a Hemi)
>> [Non-text portions of this message have been removed]
>>
>>
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>>
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Please address private mail -- mail of interest to only one person -- directly to that person.  I.e., send parts/car transactions and negotiations as well as other personal messages only to the intended recipient, not to the Clubhouse public address. This practice will protect your privacy, reduce the total volume of mail and fine tune the content signal to Mopar topic.  Thanks!

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