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Discussion Starter #1
OK, why is it that tire sizes work like this:

15x7 wheels: P205/60R15
14x6 wheels: P205/70R14

I must be missing something obvious.

Thanks,

Gary
 

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It's called plus sizing. If you have a 15" rim, you can use a 60 series tire to make the overall diameter the same as a 70 series tire on a 14" rim.

The width of the rim determines the width of tire you can install safely.
 
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205 is the crossection width of the tire (not the tread) in milimeters. To convert to inches, divide by 25.4:

205 / 25.4 = 8.07 inches.

Now the height of the sidewide is a percentage of the width, ie..

8.07 x .70 = 5.649 for a 70 series tire
8.07 x .60 = 4.842 for a 60 series tire

Being you have two sidewalls in the total height of the tire, you have to double that number and then add the wheel height/diameter for the total tire height:

5.649 x 2 = 11.298 + 14 = 25.298 for the 205/70 - 14
4.842 x 2 = 9.684 + 15 = 24.684 for the 205/60 - 15

The difference in overall tire height/diameter is 0.614"
Now as far as the width of the wheel as you have highlighted, it only very, very slightly affects the crossection width of the tire because it controls how far the beads of the tire are "spread apart". Ideally you would want to match the wheel width to the tread width, that way the tread lays flat on the ground when looking at the tire from the front of the car.
Hope this helps.
Tire Rack has some more good info on this subject. Maybe easier to understand too!

Jason
 
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Well
Yes you are
50/60/70/75/80 are the aspect ratio
This is a ratio of sidewall height to width.
 

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Discussion Starter #5
I knew i shouldnt have asked! /forums/images/icons/laugh.gif

I sort of get it./forums/images/icons/smile.gif

I wanted to know if it was important for making a tire choice for 15x7.....(any suggestions!?)

Thanks for that long, descriptive explanation. I'll read it several more times and try to understand it /forums/images/icons/laugh.gif

Thanks,

Gary
 
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15x7 wheel on your '68, I would say probably some where around
235/60-15
Tirerack's recomended rim width for this size is 6.5-8.5" on their chart for Firestones's Firehawk SS20 (these are what I have on mine)
The installed dimensions are as follows:
wheel width = 7"
section width = 9.5"
tread width = 7.7"
over all diameter = 26.1"
 
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I almost forgot above fellow VMFer jay's site he has a very good wheel/tire fittment guide for 65-70. Check him out.
 

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Gary, I have a 67 coupe, and on it I have 15 x7" rims all the way around. On the front I have 235 60 15 and on the rear, 255 60 15. The back tires are tight, but they fit and do not rub. I have a pic if you would like to see it.........Kalin
 

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Discussion Starter #9
I'd love to see it - it will mostly be for looks anyway, otherwise id be sticking with my 14"'s. Want a bit of a meaner look.

Gary
 

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Discussion Starter #11
That is exactly the way i want it! What tires are those? P225 ?

Gary
 
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