As the middle number gets bigger the tire will gat shorter AND wider given the same first number. You said it but I think you may forget the first number is not tread width it is section width, whee the sidewall bulges out. A 205/60 will be shorter and wider than a 205/70.
We seem to have two distinctly different understandings of tire widths and heights. I have to disagree with what Jeff and Jimmy, stated on this one. Also, as the middle number gets bigger, the tire gets taller, not shorter as DIYguy posted. Actually, the link Jeff posted explains it pretty much as I understand, so I may not be interpreting his post correctly.. My understanding is the first number is the tire (Metric/in millimeters) width, so that a 255 tire converts to about 10" (255 /25.4). The second number 60,70,75 etc. is the height of the tire section given as a percentage of the width. So, given the same width tire, the higher the second number, the taller the tire. Obviously the third numbed is the wheel size, so adding twice the results of the first two numbers will give the diameter. I honestly have no recollections of the old bias ply tire sizes, and I'm thinking that ty may be where the confusion comes in. ?? Do modern day tire designations even have a "series" tag? I think not.
Tex, I can't believe you didn't catch my mistake above ! As that middle number gets SMALLER the tire will be shorter and wider. This is the method I use when looking at tires, if I want more details I look at all the dimensions of each tire and compare. (I knew it was a mistake to weigh in on this one!)
Aside from the standard interpretation of the tire code, there are other factors that come into play, that I'm not sure if/when "standards" come in. What I'm talking about....obviously not all wheels are the same width, and I don't know how that affects results/calculations. Do all tire manufactures of a specific size design them to be used on the same size wheel width??, or for example, could one manufacturer design a 205-70-15 tire for a 5" wide wheel, while another manufacturer design the same 205-70-15 for a 6" wheel?? Again, I don't know, just thinking out loud. How does a specific tire mounted on a 6" wide wheel compare to that same tire being mounted on a 7" wheel? I don't know. I ***ume there is a recommended wheel width range for each tire width
LOL...I DID catch it..quote from my first paragraph: "As the middle number gets bigger, the tire gets taller, not shorter as DIYguy posted". But....your other mistake....the tire does NOT get wider. The width is set by the first number, and does not change as the height is increased or decreased by the second number.. edit, edit, edit. The widths/profile may change some, but that's why I was rambling on about wheels/mounting/etc. in the post above.
Man, I can sure ramble. A simpler way of stating all said above: 255/70/15 255= width in mm (255/25.4=10.039") 70 = height of cross section as a percentage (10.039 X .70 = 7.027) 15 = wheel dia (inner tire dia) That is a set formula. The calculated diameter for the above example is 29.055 (15. + (2 x 7.027)) Your results may vary depending on mounting or possibly manufacturer, but that does not change the formula.
Jeez, How can I just not see stuff like that! I will politely put my lid on this can of worms as it is clear I cannot explain something I don't grasp completely!
Well, let me crack the lid open a little. I checked the specs on a few tires and I was right, and then I was wrong. I will continue to check specs of individual tires. I am starting to ramble too!
Hey, until recently, I never had a clue as to what the tire designations meant. When you get to the point of trying to squeeze the biggest tire possible on the back, knowing what those designations meant made all the difference in the world. Knowing the formula allows you to figure out how to change widths without changing diameters, or visa versa. Discount tire has a great tire size calculator you can use to adjust your final results. Now, with all that said, let me know if I'm wrong, lol. (just saw your last post) http://www.discounttire.com/learn/tire-size-calculator
Exactly the same explanation as Discount tire. The info below is copied from your source. I really don't want to read their entire website, so if you have something you've found there that shows as you've stated, I'd sure like to see it. (Lea en español) Most modern p***enger car and light truck tires feature size designations that indicate the tire's dimensions in a combination of metric, mathematical and English systems. While this unusual combination of millimeters, percentages and inches is a byproduct of the evolution of global tire specifications, it also provides the ability to calculate/estimate basic tire dimensions. Example size: 225/45R17 The first three numbers in a typical size (225/45R17) are the tire's indicated section width in millimeters, measured from sidewall to sidewall. If you are familiar with measurements in inches, the section width in millimeters can be converted into inches by dividing it by 25.4. For example: 225mm / 25.4 = 8.86" The second pair of numbers (225/45R17) is the tire's aspect ratio or profile. This percentage represents the ratio of the sidewall's section height to the tire's section width. The section height can be calculated by multiplying the section width by the aspect ratio percentage. The answer will be the height of one sidewall. For example: 225mm x 0.45 = 101.3mm 8.86" x 0.45 = 3.99" The last number (225/45R17) is the diameter of the wheel in inches. If you are familiar with measurements in the metric system, the wheel diameter can be converted into millimeters by multiplying it by 25.4. For example: 17" x 25.4 = 431.8mm To calculate the overall diameter of a tire, the sidewall height must be multiplied by 2 (remembering that the tire diameter is made up of 2 sidewalls, the one above the wheel and the one below the wheel touching the ground) and add the diameter of the wheel. 101.3mm + 101.3mm + 431.8mm = 634.4mm 3.99" + 3.99" + 17" = 24.98" "[The] unusual combination of millimeters, percentages and inches is a by product of the evolution of global tire specifications, [and] provides the ability to calculate/estimate basic tire dimensions." Warning: These calculations (and those provided by all online tire dimension calculators) only reflect nominal tire dimensions, which are often subtly or substantially different than the tire's actual physical size and/or the dimensions provided by the tire manufactures. Drivers working with close fitment tolerances or trying to maintain the original tire diameter of staggered size fitments (similarities or differences) should use the tire manufacturer's specifications for the exact tire(s) they are considering. Actual Tire Section Width is Dependent on Wheel Width All tire sizes are ***igned specific rim widths upon which they are measured (measuring rim), as well as can often be mounted on slightly narrower or wider wheels (rim width range). Therefore, it is important to note that actual tire section width will depend on the wheel width the tire is mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in rim width, being reduced if mounted on narrower than measuring wheel and increased when mounted on wider wheels.
I have to chime in on this. Jimmy2car you need to read it again. A 205/75/15 is NOT wider than a 205/70/15 it is taller NOT wider. The only exception would possibly be between manufacture, but comparing same tire manufacture they would be the same width. The second # designates the aspect ratio and has no bearing on width, its the % of the sidewall height relative to width. I copied and pasted your"source" P225/50R16 The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch). The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.
lol... all this discussion to confirm that I was right in the first place. LOL I know with 4x4 tires though the bigger middle number with equal section width will be very slightly wider, but that is due to having more sidewall to balloon/wrap. the tread part would be the same width.
I'm totally laughing my *** off reading this. Trying to explain all of this is like getting your fingers caught in one of those chinese finger traps... The more stressed you get, the harder it is lol. Ok, I am going to throw my 2 cents in the ring for (hopefully) a very simple explanation... Here goes: As the middle number goes down (***uming the first number stays the same) the tire will be shorter and it will LOOK wider. It won't be any wider because the first number is the section width, which is constant. It LOOKS wider because as the aspect ratio goes down, the tire become more squared off. If you keep dropping the middle number, 75, 70, 60....45 it will completely lose the "bulge" in the side of the tire and then look like it come off a Ferrari or some kid's Honda... But... still the same width... Although... the contact patch of the tire (not shown on tire size) will become slightly wider as the number goes down. The tire gets more square ya gotta add more tread. So, in theory, if you are talking contact patch then yea, it gets a little wider... Maybe I helped... maybe I just got someone stuck in the finger trap... I like the pie crust slicks on the back of my Merc. They are 28.5" tall with an 8" wide contact patch. Oh and on a 15" wheel. Those were the numbers
Jimmy, I think you are right. I checked two mastercraft tires on summits website and a 205/70 is wider and shorter. But my theory was blown when I checked a 205/65. Should have been shorter And wider( by my theory) but it was neither. Damn that finger trap is tight!
Still probably going to be a week or two before my tires come in, so I went ahead and got some aerostar springs for the front too. Now I need to pick up some shocks. Does anyone know the Energy Suspensions part number for the coil spring pad/isolator or should I just get a rubber one, I'm pretty sure mine are toast. Also might be a good time to upgrade any bushings that I can since I'm already going to be working on the front end a bit. Nascar Dave doesn't make a sway bar for the 53' I don't believe, anyone know where I can get one?
When I wrote my statement re tire sizing, we, my friend Willie & I had just finished trying to mount both tire sizes onto his 52 Mercury: Tires: 205 75 R15 (Cooper tires) and 1 205 70 R15 (Cooper tire) Both tires on 7" rims. The 70 series would not mount without removing the shock, due to the wheel well opening, whereas the 75 would. Thus, we deemed the 70 series is slightly wider.
Jimmie, I'm curious if "both the tires on 7" rims" were the same or identical rims. Any posibility the wheels were different in their backspacing? That would account for the mounting differences.
No Tex Mine were on the Cragar rims, his were on chrome reverse No idea of any difference, but we didn't check. I wanted wider/larger tires, but didn't want any h***le in trying to change one the road.
So, if I'm understanding correctly, the 205/75/R15's were on one of those sets of wheels, and the 205/70/R15's were on the other set, and it was ***umed the reason the 70's wouldn't fit is they were thought to be wider. Is that correct? One of the the biggest factors in tire and wheel combos fitting or not fitting is the backspacing of the wheel. I now have to ***ume that is probably the cause of your fitting/not fitting. I sure understand not wanting tire changing h***les on the road and still wanting wide tires in the back. I'm currently running 235/70/15 on mine and will go to 255/70/15 on my next tire change. With thw wider tires, I'll still have about 7/8" clearance to my (stock) fender lip, and 1/2 on the inside at the springs. What I went thru on mine to make that happen: 1. narrowed the rear 3/4 per side to compliment the wheel choice I made which was not available in the ideal backspace. 2. changed to shocks that would let the differential drop an additional 2" over oem configuration 3. changed to disc brakes (easier getting a wheel over a rotor than a drum) 4. kept the rear ride height at approximately oem (which was ok because I wanted a California rake anyway)
Corrrct Tex. I like the stance of your car, but do not want to make any rear axle mods at this time. I like the way my cars sits, but wish I could have a bigger tire mainly for lower rpm on the hiway
The Jalopy Journal
