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Old 01-13-2009, 06:53 AM   #1
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Question Tire Confusion..

New to sign up for the forums, but I have been surfin' them for a while. So first I would like to say hello to everyone .

I recently bought a 2001 v6 with 16" stock rims... 225/55/16

I have bought a set of rims that are 18"-7.5. My problem is I have looked for days on here and the internet and I can't get a final answer as to what size tires to use. I have contacted local tire shops around town and they are behind the times when it comes to custom rims, so basically they don't know anything. I am trying to find the best size that not just looks good, but has tread and doesn't rub, cause it is stock suspension. I have had several sizes to consider, but I can't afford a process of elimination on tires. When I bought the rims they had 255/55/18 on them, but rubbed major, so I had them removed from the rims.

The sizes I have been quoted are 245/40/18, 235/40/18, 235/45/18. I really need someone with the mustang knowledge to lead me in the right direction so I can get a set of tires for these rims that are just sitting here for now.

I would appreciate it if someone could help me out with this
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Old 01-13-2009, 08:42 AM   #2
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Re: Tire Confusion..

First off lets take your stock tire size, 225/55/16. That is a tire with 123.75mm sidewall height. 1 inch = 25.4 millimeters and 1 millimeter = 0.0393 inch for reference. The sidewall height is the middle tire number divided by 100 times the first number. Basically its the percent of the first number.

The sidewall height for your stock tire is 123.75mm, if you went to a 17 it would need to be around 110.05mm (123.75mm is the sidewall of the top or bottom of the tire, added together that is 247.5mm of overall tire you have to account for, you need to take an inch from that so 247.5 - 25.4 = 222.1 and that divided by 2 is one side of the tire height, so 111.05mm).

That is all well in good but you want 18 inch tire information. So 247.5-25.4x2, that takes the tire down two inches. So 196.7mm / 2 = 98.35mm. That is the sidewall height you are looking for for an 18 inch tire in order to keep the overall diameter near stock. You dont have to be 100% right, you can be a little off... as a tire wears it gets more narrow anyway, its not a huge deal... just keep it close.

So 98.35mm..

235/40/18 = 94mm
245/40/18 = 98mm

Its between those two, on a 7.5 inch rim the widest tire I would personally run would be a one of those two, 10mm is not very much overall (.0393 of an inch... and remember its 5mm inside and 5mm outside... so its half that extra bulge.)

See which size is the best price and go with it.

One last note, each tire maker has its own standard of measurement... so you can buy two tires the same size and one will be wider than the other most of the time. They generally hold true to the sidewall height so don't worry, the only issues is really how wide the tire is.

Now to the rims... fitment and rubbing all depends on how they are setup, do you have any information on the rims back spacing or offset?
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Old 01-13-2009, 09:52 AM   #3
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Re: Tire Confusion..

Brandon, I believe that's the most informative post I've read in a while.
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Old 01-13-2009, 09:56 AM   #4
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Re: Tire Confusion..

Quote:
Originally Posted by SpectorV View Post
... 1mm is 25.4in and 1in is .0393mm for reference.
Something doesn't look right in that statement...

But I agree, 235 or 245's are what you should run. The rubbing could be caused by the backspacing being incorrect for a mustang. If you know the offsets and back spacings...or if you could tell us what rims it was you bought...we might be able to help out a little more.
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Old 01-13-2009, 10:00 AM   #5
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Re: Tire Confusion..

First off thanks for helping me spectorv...

I searched several websites selling this particular rim, but they didn't offer up any info on them..so I read up on how to get the specs and I measured and converted and came up with these..

Offset: 3/4" = 19 mm

Backspace: -44 mm

I hope those totals make sense, I tried

The rims are CX or Chrome Expression 810mb , 18x7.5
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Old 01-13-2009, 02:42 PM   #6
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Re: Tire Confusion..

Quote:
Originally Posted by PureVenom View Post
Something doesn't look right in that statement....
oops sorry, fixed now.

Your back spacing should be max 6 - 6.5 inches~ max. You will probably have to figure that one out for yourself, just take the rim face down and then take something flat and sit on its back across it. Measure from there down to the back of the rim where the rim will meet the rotor, thats your back spacing (dont include the items width in your measurements)

To calculate offset you'll need the following measurements:
Wheel backspace - Calculated from above
Wheel Width - 7.5 inches
Wheel Center line (outboard flange to inboard flange measurement / 2) probably wider than 7.5 inches, basically the far edges

Subtract:
Wheel center line from Wheel backspace to get offset.
If backspace is less than the wheel center line the offset is negative
If backspace is greater than the wheel center line the offset is positive

As far as fitment goes... the wheel is 19mm positive offset by your info, stock on the mustang is 24mm as far as I know, so they are close.

I dont know your wheel center line but say its 4 inches then the offset is 19mm toward the outside (front of rim) from the 4 inch mark. So... 4 inches - 19mm is 3.25 inches. That means that the rim sits past the rotor 4.75 inches.

With that said a stock 17x8 gt rim is about the same, it will sit in more of course but not much. The only issue you may run into is brake caliper clearance~ but you should be ok. Just mount the rim and see how it spins. And make sure visually you like how the rim sits, not to far in the fender.

If the rim sits in to far they make spacers for them, but not far enough... thats the only problem I see you having that cant be easily fixed.
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Old 01-13-2009, 02:46 PM   #7
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Re: Tire Confusion..

Quote:
Originally Posted by lowflyn View Post
Brandon, I believe that's the most informative post I've read in a while.
+ 2 Did you check out the infamous Zim's tire thread http://www.mustangevolution.com/forum/t25784/
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Old 01-13-2009, 04:31 PM   #8
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Re: Tire Confusion..

thanks spector...the backspace is 5-1/2"

mount to back edge of rim 5-1/2"



Quote:
Originally Posted by Herbstang View Post
+ 2 Did you check out the infamous Zim's tire thread http://www.mustangevolution.com/forum/t25784/
yes I did read it several times, but it did not apply to the 7.5 nor to my specific questions
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