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Car guys question about tire sizes

Discussion in 'The Okie Corral' started by tslex, Dec 31, 2012.

  1. tslex

    tslex

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    I need to replace the tires on my truck. I have what I acknowledge is a stupid question to anyone with more familiarity with this issue than I have -- which is likely everyone.

    Current tire size is 285/70-17. Tires in this range can be awfully expensive.

    Which of those parameters can I change in a new tire purchase? Any of them?

    Obviously the rim diameter (17, right?) isn't changing without the wheel changing and I'm not changing the wheels.

    I THINK I understand that 285 is tire width in mm. But is that the width at the rim (in which case I assume I cannot buy a different size) or the width measured somewhere else (in which case I assume I could use a different width)?

    As for the the 70 -- I do not have any idea what that number means.

    ETA: Maybe a simpler question: Can I replace the current 285/70-17 tire with a 265/70-18 tire on the existing wheel?

    (I get it -- if you understand this stuff it sounds like asking if i can shoot .45ACP out of my 9mm. But I'm ignorant in this topic, so. . . _


    Thanks in advance to the GT braintrust.
     
    Last edited: Dec 31, 2012
  2. Glock30Eric

    Glock30Eric .45 ACP

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    Tagged to get educated.
     

  3. travisstorma

    travisstorma

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    285 is the tire width at the tread. The 70 is the height as a percentage of the width.

    285 mm with
    285 x .7 = 199.5mm tall sidewalls on either side of the rim.
    The 17 is the rim diameter.

    Your overall tire size would be 285mm by 199.5mm + 199.5mm + 17 inches. This would be roughly 11.2 by 32.7 r 17 tire.
     
    Last edited: Dec 31, 2012
  4. travisstorma

    travisstorma

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    No. The 17 has to be the same.
     
  5. Dan_ntx

    Dan_ntx

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    Percentage of height to width.

    285 = width in mm
    70 = % of height to width (70% of 285mm tall)
    17 = rim size

    Remember that 25.4 mm= 1 inch when doing your calculation

    Or use a tire size calculator on the interweb
     
    Last edited: Dec 31, 2012
  6. MtBaldy

    MtBaldy Obie Wan, RIP

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    Not an expert but have gone through some of this looking for performance tires. The 285 is the TREAD width. The 70 is the aspect ratio. The 17 is as you surmised the wheel diameter. Usually if you change both aspect ratio and tread width you can come close to keeping the same wheel/tire diameter which means your speedometer will stay close. For instance if you go up a size in tread width you go down in size on aspect ratio. As an example only, if you go to 295 tread width you would go down to a 60 aspect ratio. Since the rubber is flexible and under pressure you have some flexibility with the rim. You need to know the rim width and how close your current tires are to it's maximum, or minimum, tire size your current tires are. Finally, my main source of information was here:

    http://www.tirerack.com/FAQ/index.jsp
     
  7. tslex

    tslex

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    Thanks. I see that. 17 is the diameter of the rim, so that's fixed. Got it. Does the 285 have to be the same? Because that seems to be the biggest effect on price point, all other things equal.
     
    Last edited: Dec 31, 2012
  8. MtBaldy

    MtBaldy Obie Wan, RIP

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    No, the tread width and aspect ratio are where you have some flexibility.
     
    Last edited: Dec 31, 2012
  9. tslex

    tslex

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    Baldy, thanks! That was my core question I guess, whether the diameter was the diameter at the rim or somewhere else (tread per your your response).

    Cost difference can be $50/tire going from 285 to 265.

    Truck spends 90 percent of its life on the pavement (but often in tons of rain), 7 percent on unpaved roads, and 3 percent off road, but mostly in fields and at the range.

    I've learned over 135K miles that the best combination is a moderately aggressive tread, but with deep siping for all the rainy pavement I drive on.

    Thanks to GT GNG for a quick response. Any general advice on tires also welcome.
     
  10. NAS T MAG

    NAS T MAG

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  11. skinny99

    skinny99 Crew Chief

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    Depending on what kind of truck you have you could change the width 285 or the Aspect ratio 70. However if you want the tire to remain the same size it will be very tough to alter either. Even going slightly larger ups the price considerably. I would never go smaller, looks like crap and hurts fuel mileage and ride quality. 285/70-17 is a very popular tire and will be the cheapest way out.

    One option you may try is craigslist or your local performance wheel and tire shop. Someone may be selling some low mileage "takeoffs". Lots of guys take off almost new tires and wheels to put bigger wheels and tires on. Of course the downside is no warranty on balance or trueness of the tire. Just a suggestion.
     
  12. Dan_ntx

    Dan_ntx

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    Just use a tire size calculator or manually calculate the total height and get close to what you have currently so you don't change your final drive ratio or make your speedometer inaccurate.

    Another consideration is tire width to wheel width. If you make your tire too wide or too narrow for the rim you will have issues, however you can generally add or subtract 30mm of width with no impact...just make up for the total height by changing the aspect ratio.

    Discount tire's website will suggest alternate sizes BTW, I'm sure others will too.

    If you are looking for inexpensive tires, have you looked at Treadwright? Good quality retreads that are popular in the Jeep and off road community. I know lots of guys who run them trouble free on their Jeeps.
     
    Last edited: Dec 31, 2012
  13. CitizenOfDreams

    CitizenOfDreams

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    My 2 cents worth of tire advice: unless you really know what you are doing and what are you trying to achieve, it's probably best to stay with the factory recommended size and load rating.

    If you are on a budget, there is some room to choose between more expensive and less expensive brands/models. But changing the tire size just to save money is most likely false economy.
     
  14. wrczx3

    wrczx3 Jerry

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    The closest size I could figure out would be a 255/75/17. The would be 1/10 of an inch shorter and 30mm (1.18") narrower. I know nothing about guns but sold tires for 20 years.
     
    Last edited: Dec 31, 2012
  15. itstime

    itstime

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    285 is section width. Not tread width
     
  16. I'M Glockamolie

    I'M Glockamolie

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    What is the year, make, model of truck? Try tirerack.com to see if you have any better options in the stock size.
     
  17. wrczx3

    wrczx3 Jerry

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    Don't forget to look at the load index and speed rating.
     
  18. pipedreams

    pipedreams Member

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    This................

    Your don't want to go with smaller than the factory originals. Lots of good brands other than what the factory puts on. When I put new tires on my truck someone suggested Kumo tires and I got a good price and they seem to be working out great after a year. Check around and I'm sure you will get some good recommendation here on the forum.
     
  19. LEO/Dad

    LEO/Dad Navy Veteran

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    Many people don't realize that your vehicle's recommended tire size is on your door jam label along with recommended inflation pressure. If there were ever any liability issues, I would have to assume that this would be checked!
     
  20. wrczx3

    wrczx3 Jerry

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    You are right. A lot of tire shops will not replace your tires with anything but the factory size. Any shops I have worked in will as long as the new tires are very close to stock size and the load index and speed rating are the same or higher on the replacement tires.
     
    Last edited: Dec 31, 2012