Tire size calculator November 12, 2006, 09:23:40 AM 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 Quote Selected
Tire size calculator Reply #1 – November 13, 2006, 09:29:40 PM Not advertising for this company but heres another onehttp://www.tirerack.com/index.jspFirst go to tire by sizeThen select a tireThen click specsWhat’s nice is you can see different company make the same tire but the dimensions are different. Quote Selected
Tire size calculator Reply #2 – November 13, 2006, 11:36:56 PM QuoteSpecification Sidewall Radius Diameter Circumference Revs/Mile Difference225/60-15 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. :D Quote Selected
Tire size calculator Reply #3 – August 12, 2007, 11:49:29 AM 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. Quote Selected
Tire size calculator Reply #4 – September 14, 2008, 04:27:37 PM Ya it is a great tool. Quote Selected
Tire size calculator Reply #5 – September 14, 2008, 06:09:16 PM Quote from: Sick88Tbird;168189It'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... Quote Selected
Tire size calculator Reply #6 – September 20, 2008, 09:22:42 PM Quote from: V8Demon;236163It'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. Quote Selected
Tire size calculator Reply #7 – September 20, 2008, 10:54:24 PM Quote from: Sick88Tbird;168189It'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. Quote Selected
Tire size calculator Reply #8 – September 21, 2008, 09:39:44 PM Quote225/60r15 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.... Quote Selected
Tire size calculator Reply #9 – August 23, 2009, 10:59:03 PM 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. Quote Selected
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