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Need some help with tire sizes

5K views 23 replies 5 participants last post by  Vicious013 
#1 ·
Hey guys, i need a bit of help with my tire sizes. So when i bought my car second hand it came with 20" rims on it and the tires were as following : 245/50/20 in the front and 275/45/20 in the rear. Now this caused some problems for me because i couldn't turn the steering wheel all the way due to the large tires in front. I swapped them out with 255/35/20 all around.

Now i want to get a new set of rims and tires and i'm thinking to go 19" because 20" is just a bit tough on the road, every pothole or small imperfection on the street just shakes my whole car and basically need to crawl around these areas, especially cubic rock roads which i have to drive on one to work every day, it's terrible.

So my question is, what kind of tire size would be best ? The rims i'm looking at are 8.5 in the front and 10 in the back so i would want some tires that would give me a staggered look and also not to have the problem i had before with the front wheels not turning to the max ?

If i would go for 20" rims still i was thinking to go 255/35/20 in the front and 275/40/20 in the back (8.5 and 10 rims) but if i should choose the 19" version what sizes would fit better ? Can i go over 35 in the front and still be able to properly turn my steering wheel ?
 
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#2 ·
sounds like a good time to go back to basics on tire and wheel sizing and fitment:

For clearance, you need to be concerned with:
> the overall diameter of the tires
> the width of the tires
> the offset of the wheels . . . the more offset, the more the wheel/tire is tucked inside the wheel well; the less, the more it sticks out

If you stay within the "envelope" of the original tires/wheels, you can be sure that you will have no clearance issues. Or you can go a bit bigger, but that's when it gets less clear about what you can get away with.

for the S197 GT's, I'm pretty sure that the original tire size was 235/50-18. You can look your year up on Tire Rack to make sure of the original size; you can look up all of the tire size information, for a specific brand/model/size of tires, on Tire Rack also.

Assuming the standard size, the overall diameter was 27.2-27.3 inches. Now that you know that, you can look for sizes that are about the same; from Tire Rack "sizes" tables, here are some examples:

https://www.tirerack.com/tires/tire...8P0ASPXL&vehicleSearch=false&fromCompare1=yes

245/40-19 -- diam = 26.7" (a little small)
255/40-19 -- diam = 27.0" (a hair small)
275/40-19 -- diam = 27.7" (a little big, but this is a commonly used size and I know it will clear because I have them on my car)

Next you need to make sure the tire width is correctly sized for the wheels. You can check this on Tire Rack also; look at the "rim width range." Generally 235 or 245 is about the widest you can go on an 8.5" wheel; I would stick with 235. For the 10" wide wheels on the back, 275 or 285 is generally good. This does vary a little by make and model so the best plan is to look up the specific tires and check the rim width range and also the "measuring width" which is close to ideal for that tire.

Regarding the wheel offset, if you are buying the wheels from someone who has good Mustang fitment information, they should have that covered.

One more thing to think about if you are buying wheels: do you want the front wheels to clear big brakes? If you think you might upgrade your brakes in the future, this would be a good time to make sure that the wheels you are buying will clear them. Generally 9" is the narrowest that will clear the Brembo brakes, plus they need to have the right spoke design (spokes must be out toward the edge of the wheel, cannot curl back inward like Bullitt wheels)
 
#3 ·
Are we talking V6 or GT? The Avatar shows GT fogs on the front but the profile comments say V6.

The V6 has more turns on the wheel to reach hard stop vs the GT.

Placing too wide a tire on the front of a V6 means you risk rubbing tires at full turn that wouldn't otherwise rub on the GT.
 
#4 ·
Is there a sticker on the drivers door jamb with the tire size? See picture for an example, this will get you back to basics. If it is an 18" original tire size then you can use Tire Rack or an online tire calculator to find the 19" equivalent to the 18" as far as over all diameter to keep your speedo correct.


https://tiresize.com/calculator/
 

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#5 ·
@JBert thanks for the answers, i guess having 245/50/20 was too big for the front wheel. you mentioned on a 8.5 rim the largest could be 235 - 245/40/19 right ? what about 20" can i go 255/40/20 on a 8.5 rim in the front ? i am not planning on changing the breaks soon, i think the last inspection i had was 1 month ago and the guy said they are in very good condition.

@ONEZ ST it's a V6, the fog lights are after market. I did had the issue with turning the wheels with a set of 245/50/20

@my89foxbody i don't think i have that sticker, the previous owner must of peeled it off

I can see on tyre rack the sizes for 19" are : 245/45/19 - front and 275/40/19 - rear. Why is the rear aspect ratio smaller than the front if the wheel doesn't need to turn ?
 
#6 ·
@JBert thanks for the answers, i guess having 245/50/20 was too big for the front wheel. you mentioned on a 8.5 rim the largest could be 235 - 245/40/19 right ? what about 20" can i go 255/40/20 on a 8.5 rim in the front ? i am not planning on changing the breaks soon, i think the last inspection i had was 1 month ago and the guy said they are in very good condition.

@ONEZ ST it's a V6, the fog lights are after market. I did had the issue with turning the wheels with a set of 245/50/20

@my89foxbody i don't think i have that sticker, the previous owner must of peeled it off
An 8.5" wheel could also have 245/45R19 as well, that is what my stock size is on a '14 GT Premium
 
#13 ·
The standard tire size on the '05-'09 V6 is 215/65R16 which is 27.0" in diameter.
The 255/45R18's that I'm running on my GT are exactly the same diameter. You could go staggered with 255/45R18 front and 285/40R18 rear. If you prefer 19" rims instead of 18", you could go staggered with 255/40R19 front and 285/35R19 rear.
The 20" rims you currently have leave too little tire in order to maintain a stock 27.0" overall diameter so I suggest you sell them.
 
#14 ·
Thanks for the info, what would you say is the maximum you can go on 20" for staggered ? I've had a quick chat with someone from a tire shop website and he said with 255/40/20 on the front 8.5" rim then the 275/40/20 on the rear with 10" rim would run without any issues. Perhaps even a 285/40/20 on the rear ?
 
#16 ·
^ ^ ^ but it won't get you away from the harsh ride that you described in the first post; to do that you will need to go to 19" or even 18" wheels . . . to get a decent amount of sidewall you probably really want 18's
 
#17 ·
yeah i think i'm just going to live with that, it was all right so far with 20" rims, a bit rough on some portions but i guess it is like that with wheels this size.

thanks a lot for the replies guys, lots of useful info, i do have one more question, sorry for being so noob but i want to be 100% sure before i spend what it is to me a lot of money.

On a set of 20' rims, 8.5 front and 10 in the rear, is it possible to go 255/40/20 front and 285/40/20 back ? Or is it too much and won't have wheel clearance ? Quite important for me if these sizes will fit right or not :smile:
 
#19 ·
attached images show what you would find if you looked up those sizes

we don't know for sure if 28" diameter will clear on the front; all we know for sure is that 27" will clear (plus you will want the new steering bump stop due to the added width)
 

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#21 ·
. . . car second hand it came with 20" rims on it and the tires were as following : 245/50/20 in the front and 275/45/20 in the rear. Now this caused some problems for me because i couldn't turn the steering wheel all the way due to the large tires in front. . . . Can i go over 35 in the front and still be able to properly turn my steering wheel ?
Not sure about the steering bump stop, right now i have 20" wheels and they came like this from the previous owner. I have no idea if he added something or not but so far i haven't had any issues with these wheels.
I thought you said you had rubbing problems in the front when you turn the wheel all the way? That's what the steering bump stop is for.
 
#23 ·
aha OK . . . so the rubbing was not caused by the tire width, it was the huge overall diameter of the other tires (plus the width)

I'm pretty sure the GT steering stop is needed when you put wider tires on the V6 so I guess that was already done or is not needed in your case
 
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