New Tires on stock 17" Rim...can somebody confirm my math?

[Redleg]

I'll be picking up my 2007 Mustang GT with the optional 17" bright cast aluminum wheels soon. My question is...what is the widest tire I can run on this rim? As I understand it...now all 17" rims are created equal. Is this a 17x8? 17x8.5? I've searched the Ford site and couldn't find the reference...can anybody clue me in/show me where they got this type of data?

 

I'm also concerned about height of the tire. The stock tires are 235/55. From what I understand...the tire is 235mm wide from sidewall to sidewall. The .55 aspect ration means that the tire is 129.25mm "tall" from edge of rim to tread of tire. Am I doing this math right?

 

Assuming it's correct...

 

If I wanted to go to a 275/40/R17 tire...that would make the tire 275mm wide and 110mm tall (using the same math from above)....in other words...the tire on 275/40 tire on the rim would not sit as tall as the stock tire?

 

Same concept goes for a 285/50/R17 tire....285mm wide and 114mm tall...again not as tall...just wider.

 

Now going back to the top...can the stock 17' rim take that wide a tire? What's the process to find out?

 

Looking into the future...my tire selection is going to be important for when I lower my ride with either the Steeda or FR Handling Pack. I don't want to have too big a tire that can't accomodate the setup.

 

Any help is appreciated. Thanks.

[Five Oh B]

Redleg, the 17" wheels are 8" wide. The factory 235/55/17 tire is 27.2" tall. My 2000 Mustang GT and my 2003 Mach 1 both came with 17" x 8" wheels and came with 245/45/17's on them. I put 275/40/17's on the the back wheels (against my better judgement) and they worked, but the sidewall flex made the tire wobbly feeling around corners, but straight line traction was better with the extra width. I finally decided it was better to get the correct wheels under the car, so my Mach 1 now sports 17" x 9" wheels from a 2003 SVT Cobra and 275/40/17 rubber at all four corners. The car handles much, much better and traction is great. Overall weight went up a bit, so acceleration was affected minutely, but makes for a much better/safer driver.

 

The widest tire that should safely be used on an 8" wide wheel is likely a 255mm width. Using a 50 series tire (255/50/17) would yield a tire that's 27.0" tall. If you use a different aspect ratio, but keep the 255mm width, it is very easy to reprogram the tire size into a handheld tuner (diablosport, sct, etc.).

[Redleg]

Five Oh,

 

Thanks for the info. How do you know the sotck 17" rim is a 17x8? Not doubting you...just wanting to know where I can look up information like this on my own in the future. Do you know what the stock 18" option is? Is it also 18x8?

 

You telling me the stock 235/55/R17 tires are 27.2 inches tall proofs my math. Thanks. I don't know if you took a tape measure to the wheel/tire or used math...but I'm assuming that the conversion comes like this (for all those that might be attempting the same calculations in the future):

 

235mm x .55 = 129.25 (length from top of rim to tread)

129.25 x 2 = 258.5 (for top of tire and bottom of tire)

258.5 / 25.4 (conversion of mm to inches) = 10.177 (amount of rubber in inches, top and bottom from edge of rim)

10.177 + 17 (rim size) = 27.17 (approx 27.2)

 

If anybody smarter than me wants to correct my process...let me know if I messed something up.

 

Next question...how do you know that a 255mm tire is the max you should go on a 8" wide rim? Is that trial, error, and experience? Or is there another math formula I can use to figure that out? I'm not seeing the relation of a 10" wide tire (255mm) on an 8" rim.

 

Thanks so far

[Five Oh B]

Your math is correct. I've always done it in a little different order, but we're on the same page. Discount Tire's website has a page to do the math for you - click here.

 

I work for a Ford dealership (same one for 7 years), have owned 5 Mustangs, I order all of our new Mustangs, etc. The new wheels are 17 X 8 or 18 X 8.5, it says so in the Ford Parts/Accessories Catalog.

 

Read the forums here and at other websites about other Mustang owners' experiences with tire widths on a variety of OEM Mustang wheels. A 275mm width tire is too wide for an 8" wide wheel per the group consensus. I've tried it myself as I have already mentioned with less-than-desirable handling. 265mm width tires are few and far between. 255mm width tires are more common.

 

The factory spends a lot of time testing tires and wheels for the ever-finicky consumers and Ford doesn't go past 245's on an 8" wide rim. For 275's (i.e. 2003-4 SVT Cobra) they upsized to a 9" wide wheel. All the companies that offer deep-dish wheels to accomodate 315/35/17's for the 1999-2004 Mustang use a 10.5" wide wheel. These are all popularly accepted wheel widths for these size tires. The 2005-7 Mustangs use a different back-spacing for the wheels, but rim widths (and appropriate tire sizes) are the same. The 2007 Shelby GT500 uses 18 X 9.5 wheels at all four corners, with a smaller 255/45/18 tire in front and 285/40/18 tire in the rear. The wider rears were Carroll Shelby's personal idea to offer more traction against the 500hp motor, and he likely would have spec'd wider tires if wider wheels would have been in the plan.

 

Lastly, don't confuse the section width of a tire with the wheel width. A wider section width than wheel width is normal (that's why the tire's sidewall bows out a bit). The contact patch is a more direct comparison to the wheel width. I don't know of a formula to predict the optimum tire width for a given wheel width, but the OEM factory and aftermarket companies spend a lot of research money to come up with ideal ranges of tires sizes for a given wheel width. I'd stick to their recommendations to be safe.

[Redleg]

Thanks a lot. That's a wealth of information. So if I want to have as much rubber on the road as a Shelby GT500...I need to go to an 18x9.5 rim. I really like the look of those wide tires.

 

Hopefully last question...is there a smart way to estimate what affect lowering my Mustang will have if I run the 18x9.5 rim with wide (Shelby size tires)? Doing the calculations, the tires are wider but shorter than the stocks so I wouldn't anticipate a problem...unless there is something I'm overlooking.

 

Thanks