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Topic: Tire size calculator (Read 12862 times) previous topic - next topic

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 :birdsmily: 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 ♣

Tire size calculator

Reply #1
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.

Tire size calculator

Reply #2
Quote
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. :D

Tire size calculator

Reply #3
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


Tire size calculator

Reply #5
Quote from: Sick88Tbird;168189
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!

Tire size calculator

Reply #6
Quote from: V8Demon;236163
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

Tire size calculator

Reply #7
Quote from: Sick88Tbird;168189
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 :birdsmily: 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 ♣

Tire size calculator

Reply #8
Quote
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!

Tire size calculator

Reply #9
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.
:birdsmily:
(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.