Results 1 to 10 of 10

Thread: Tire size calculator

  1. #1
    Join Date
    Oct 2004
    Location
    Middle Sackville, NS, CANADA
    Posts
    9,827
    Feedback Score
    6 (100%)

    Default Tire size calculator

    Somebody posted this on NATO and I thought it would be a good addition to this site. It's actually based at a Miata site, but tires is tires:

    http://www.miata.net/garage/tirecalc.html
    2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

    1988 5.0 Thunderbird SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

  2. #2
    Join Date
    Jan 2005
    Location
    long island
    Posts
    1,353
    Feedback Score
    2 (100%)

    Default

    Not advertising for this company but heres another one
    http://www.tirerack.com/index.jsp
    First go to tire by size
    Then select a tire
    Then click specs
    What’s nice is you can see different company make the same tire but the dimensions are different.
    Billy

    My motor missing the gt40 tubular upper http://foxtbirdcougarforums.com/showthread.php?t=17223
    Picks of my car http://foxtbirdcougarforums.com/showthread.php?t=17369
    http://www.cardomain.com/ride/2395591
    Car Domain will be under constrution for a while(it took me a year to do this)

  3. #3
    Join Date
    Sep 2006
    Location
    Down Under and loving it
    Posts
    1,201
    Feedback Score
    6 (100%)

    Default

    Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
    225/60-15 5.3in 12.8in 25.6in 80.5in 787 0.0%
    275/50-15 5.4in 12.9in 25.8in 81.1in 781 0.8%
    I think I did ok.

  4. #4
    Join Date
    Aug 2005
    Location
    Aiken, SC
    Posts
    1,282
    Feedback Score
    7 (100%)

    Default

    It's ashame these tire size calculators are 100% worthless...go measure a 225/60r15 while it's on your car at the recommended tire pressure...it will measure 24.5" in diameter.
    Project 3G: Grandpa Grocery Getter-'85 Crown Vic LTD 2-door, 351W with heavily ported/polished GT40 heads, heavily ported/polished Typhoon Power Plus upper & lower intake, Comp Cams 265DEH retarded 1*, FAST EZ-EFI, HD T5, 8.8" 3.73 trac lock with extra clutches, 3G alt. swap, '99 CVPI front brakes, '09 CVPI rear disc brakes, '00 CVPI booster&m/c + wilwood adj prop valve.

    Parted & Gone-'88 T-bird Sport, 351W swap, ported GT40 heads

  5. #5
    88TurboCoupeman01 Guest

    Default

    Ya it is a great tool.

  6. #6
    Join Date
    Oct 2004
    Location
    Farmingdale, NY
    Posts
    9,052
    Feedback Score
    18 (100%)

    Default

    Quote Originally Posted by Sick88Tbird View Post
    It's ashame these tire size calculators are 100% worthless...go measure a 225/60r15 while it's on your car at the recommended tire pressure...it will measure 24.5" in diameter.
    It's good for getting an idea of how close 2 different sizes are...
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

  7. #7
    Join Date
    Aug 2005
    Location
    Aiken, SC
    Posts
    1,282
    Feedback Score
    7 (100%)

    Default

    Quote Originally Posted by V8Demon View Post
    It's good for getting an idea of how close 2 different sizes are...
    It gives you a vague idea at best...and should absolutely not be used when figuring for MPH after changing gears.
    Project 3G: Grandpa Grocery Getter-'85 Crown Vic LTD 2-door, 351W with heavily ported/polished GT40 heads, heavily ported/polished Typhoon Power Plus upper & lower intake, Comp Cams 265DEH retarded 1*, FAST EZ-EFI, HD T5, 8.8" 3.73 trac lock with extra clutches, 3G alt. swap, '99 CVPI front brakes, '09 CVPI rear disc brakes, '00 CVPI booster&m/c + wilwood adj prop valve.

    Parted & Gone-'88 T-bird Sport, 351W swap, ported GT40 heads

  8. #8
    Join Date
    Oct 2004
    Location
    Middle Sackville, NS, CANADA
    Posts
    9,827
    Feedback Score
    6 (100%)

    Default

    Quote Originally Posted by Sick88Tbird View Post
    It's ashame these tire size calculators are 100% worthless...go measure a 225/60r15 while it's on your car at the recommended tire pressure...it will measure 24.5" in diameter.
    Measuring the tires wouldn't be much more accurate (if at all). The tire would get smaller as it wears. The difference in diameter between a new and a half worn tire would probably be 1/2 to 3/4 inch. The tire would also be shorter in diameter measuring top to bottom than it would be measuring side to side, probably by 1/2 inch or more. A Goodyear may be half an inch taller or shorter than a BF Goodrich, which may be half an inch bigger or smaller than a Firestone.

    Like Paul said, a calculator is good to see the difference between tire sizes. If there is a margin of error it can only be assumed that the margin is equal for all tires, therefore when the calculator tells me that there is only a 0.1% difference between a 225/60R15 and a 245/50R16 I would feel comfortable yanking the 10-holes and installing a set of snowflakes with those tires, knowing the speedo would still be accurate.

    Even if the calculator is off by 5% from the true tire diameter (the difference between the 25.6" that the calculator shows for a 225/60R15 and the 24.5" figure that you give is only 4%), that's still within the specs of most factory speedometers. It's certainly adequate for Ford speedo gears, since they're only available by tooth count and each tooth would represent several percentage points in difference. Without putting the car on a dyno and calibrating the speedometer with a potentiometer there likely wouldn't be any way of getting a truly accurate speedometer.
    2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

    1988 5.0 Thunderbird SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

  9. #9
    Join Date
    Oct 2004
    Location
    Farmingdale, NY
    Posts
    9,052
    Feedback Score
    18 (100%)

    Default

    225/60r15 while it's on your car at the recommended tire pressure...it will measure 24.5" in diameter.
    Almost forgot.....My 235 60/15's are said to be 26.1" new......
    Got over 25 1/2" on the measurement the other day. Not sure exactly how much more as it was just a quick eyeball, but you get the idea....
    -- 05 Mustang GT-Whipplecharged !! -- 87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's! http://www.fquick.com/V8Demon

  10. #10
    Join Date
    Jan 2007
    Location
    Birmingham, AL
    Posts
    309
    Feedback Score
    0

    Default

    Door sticker on my 86's say P205-70-R14 recommended tire size. I put 215's on it just to get that tiny bit more foot print on the pavement for handling. I recently bought a new Garmin GPS, which of course will tell me speed/heading. According to the Garmin, my speedo is VERY accurate with the new TigerPaws on it. I haven't found either to be more than 1 mph off at any speed. Both being digital devices, accuracy is good to +/- 1 of the last digit on either.

    I did a similar calculator in Excel to figure out engine RPM going backward from tire size, dif ratio, tranny ratio. Last I checked it, it was within 20 RPM on three different vehicles with three different tire sizes. The majority of the diameter is NOT going to change, and that would be the wheel size. Width/aspect ratio shouldnt' vary much on tires, otherwise, it would have to be a different aspect ratio, right? However, I've seen some with a very wide width, but a relatively narrow tread width, which totally screws the aspect ratio. The lower the aspect ratio, though the better the accuracy is going to be because of less side-wall flex, and again, the fixed wheel diameter. The most accurate measurment will be to record actual rolling distance of a given tire on a given vehicle. Take some kid's sidwalk chalk and make a mark on the tread of the tire, then drive on a smooth surface. Measure the distance between the marks. THEN, you can figure out what your gear change is going to do to the speedo and tach.

    Don't even get me started on newer 17 inch rim tires that inflate to 48 PSI. They make for good fuel mileage, but GEEZ they wear quick.

    (X2) '86 Thunderbird, 3.8L CFI, C5 Tranny

    '92 F-150, 5.0L EFI (SD), M5OD Tranny, 3.08 Dif

    '70 VW Beetle, 1780cc, twin Solex 43's.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •