Discussion in '1998 - 2011 Ford Ranger' started by Ryan_obx, Mar 15, 2019.

  1. Ryan_obx

    Ryan_obx New Member

    i have a 2004 ranger fx4 with a roughcountry torsion key leveling kit the stock tires on the truck are a 245/75r16 and there is a huge space in the fender between the tire and the truck an im looking to fill that space but i dont want much rubbing at all will a 285/65r18 fit the truck im not to worried about rubbing the torsion bars on the inside because i have aftermarket rims ordered with a -12mm offset and an inch and a half wheel spacer on the .way i want 18 inch rims no matter what just noy sure how big i can go on the tire before running into problems
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  3. DeanMk

    DeanMk Member

    The formula for calculating tire height is...…….

    ...the tire's listed size.

    The stock size is 245/75-R16
    Divide 245 by 25.4. This will give you the "inch" equivalent to that metric measurement (its in millimeters, btw).
    The next number (75) is the aspect ratio. This is the relationship of the sidewall's height to the tire's width (the "245" you just converted).
    It is saying is that the measurement of the sidewall's height is 75% of the measurement of the tire's width, thus, you multiply the inch number by ".75" (same as "75%").
    The last number is the diameter of the wheel in inches (and why you convert the first number to begin with. Now everything you figure out is in inches)., it works like this....

    245/25.4 = 9.645669291338583"
    9.645669291338583 x .75 = 7.234251968503937"
    7.234251968503937 x 2 = 14.46850393700787" (multiply by 2 since the sidewall is measured twice when measuring the height of a tire...think about it)
    14.46850393700787 + 16 = 30.46850393700787"

    That last number is the calculated height of the tire.
    Because of the nature of tire rubber, no two tires are going to measure out exactly the same (although P235/75-R15's hold remarkably close for some odd reason).
    For figuring out things like engine RPM @ road speed, or matching different tire heights to different gear ratios, you could probably use 30.5" and likely hit it close enough.
    If you want to know what size tire would work with an 18" rim, simply start with that overall height and subtract "18", then divide by 2, then divide by ".75" and finally, multiply by "25.4".
    The resulting number will give you the first number in the new tire size (it will replace the "245", if that makes better sense).
    This will give you an idea of what size tire you'll be looking for.
    If your truck has ABS, you'll also have to likely change the gear ratio as well as the tires, because the change in tire height that you eventually will end up at will fool the engine's computer into thinking you're going slower than you actually are and the ABS system will then act differently as well.
    ...this is the down side of all that technology. Can't just slap a set of tires on it and go down the road. The whole thing's integrated now...have fun! =)

  4. DeanMk

    DeanMk Member

    Sorry Ryan, this stupid forum ( :mad: ) won't let me correct a mistake I made in my prior post, so I'll have to correct it here.
    Towards the end of that response, I wrote; "This will give you an idea of what size tire you'll be looking for.".
    What I meant to write was...that will give you an idea of what size tire you'll be STARTING FROM, when determining just how much larger your new tires will eventually be.
    Everything else I wrote is correct.
    Hope that wasn't too confusing.


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