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Author Topic: Mathematical formulas for tire fitments  (Read 1193 times)

Leaburn Patey

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Mathematical formulas for tire fitments
« on: November 01, 2008, 12:37:31 PM »

As discussed by some members on the Friday night chatbox we were scampering searching for the formulas to answer most commonly asked questions about tire fitments and conversions.
Despite the "light truck" application,the calculations still work for any car.I use them almost everyday at my work.
CBarge2008-11-01 16:53:11
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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #1 on: November 01, 2008, 12:42:05 PM »

Another calculator I use is this:
http://www.miata.net/garage/tirecalcold.html
quick and easy to use.But cannot convert old alpha-numeric sizes to P-metrics
 
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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #2 on: November 01, 2008, 12:43:15 PM »

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1973 Satelitte wagon
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2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #3 on: November 01, 2008, 12:43:59 PM »

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Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #4 on: November 01, 2008, 12:44:54 PM »

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1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #5 on: November 01, 2008, 12:51:17 PM »

From the dark side tech index the speedo teeth chart:
http://www.moparts.org/Tech/Archive/auto/12.html
 
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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #6 on: November 01, 2008, 12:56:28 PM »

I use the Coker tire site to get the O.D. of the old tire sizes when converting to modern radials:
http://www.cokertire.com/
For example I click in H78-15 then click on details.I get an OD of 28.36
That can convert to a 2357015 radial for a Fusey,for example,which has an OD of 28.0.
CBarge2008-11-01 17:00:21
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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

firedome

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Mathematical formulas for tire fitments
« Reply #7 on: November 01, 2008, 05:06:17 PM »

Thanks Lea - very useful!!!

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Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #8 on: November 02, 2008, 02:29:32 PM »

OK...I had to do some digging and found this chart in my 1968 Technical Training book from Chrysler
It shows which speedo gear to use with what tire size.
Again,you will need to convert the old style sizes to today's P metric sizes to determine which gear to use..

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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #9 on: November 02, 2008, 02:32:36 PM »

As you see,the color of the gear does not determine its size since there are several different ones the same color.The number of teeth is on the side of the gear,as stated in the article.

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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Leaburn Patey

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Mathematical formulas for tire fitments
« Reply #10 on: November 02, 2008, 02:45:12 PM »

An example is say your car was originally equipped with a 8.55-14 tire and you are going to a modern 15" radial and swapped out the cruiser 2.76 gears to 3.55's
8.55-14 has an O.D. of 28.1
23570R15 has an O.D. of 28.0
Originally the trans would have had a Red 26 for the speedo.
Follow the chart for 8.55 it determines that you will need a Green 34 to have a correct speedo reading .
If are installing 3.23's you install a Green 31.
Both same color BUT different number of teeth.
CBarge2008-11-02 19:47:53
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1968 Newport Custom project BOAB
1973 Satelitte wagon
1983 Dodge 400
2006 300C HEMI!!

Mike

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Mathematical formulas for tire fitments
« Reply #11 on: November 02, 2008, 09:06:17 PM »

Hey Lea tell them Hosiers to give you a raise, you must have excellent customer satisfaction ratings!
FYI, I rock my Jiffy Auto Service hat here in Joisey! Its a sweet hat.
This is usefull information for those who are not sure what to put on
there old rides. I know I learned a thing or two.

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Rich

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Mathematical formulas for tire fitments
« Reply #12 on: November 03, 2008, 08:02:32 AM »

Here is a good site to use to try out different tire sizes to see which best fit your needs: http://www.1010tires.com/TireSizeCalculator.asp
 
For example my 68 Newport came with 8.55x14" wheels, and I used Coker Tires web site to find out that the modern equivalent size is 225/74x14".  I put on front dics brakes from a 73 New Yorker and so needed 15" wheels, I plugged in about a gazillion different sizes into the 1010tires.com  calculator and found that 235/70x15" are within the recommended 3% difference, and my speedo will read 58.5mph when going 60mph.  For the rear tires I used 265/60x15" which are even better, showing 59.4mph.
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