XJHEAD;180556 wrote:
Your fucking kidding right???

What do you not understand???.......

Obviously reading and comprehension was not taught in his school. Hell, even my wife understands it. Fuck, my dog practically does.

killer69penguin;180542 wrote:
again all these things are helpful but still dosent explain the reason that the spedometer would change if i put a 235/45/17 on instead of a 245/45/17

Your fucking kidding right???

What do you not understand???

Going from a 245/45 to a 235/45 the tire diameter is smaller. Le'ts say you drive 1,000 rpm's in drive(1:1) tranny ratio. and you gears are 1:1 your tire will spin 1,000 rpm's too. So to make it easier to see let say the tire diameter is 24". Then at 1,000 rpm your tire will travel 1,000PID = 1,0003.142=6280' in one minute.
Now lets say you put some 4' diameter tires on there.
1,000PiD= 1,0003.144=12,560' in one minute. Your speedo is not measuring the the distance your tire travels but it measures the speed of the tranny output shaft(or whatever on rice??) that is why on some you change the gears on the speedo to correct gear or tire size changes.......

plug and play what you are looking for, then scroll down for some pretty descriptive details of your selections...

the distance a tire rolls in one turn is roughly the same as the circumference (distance around) of the tire. the circumference of a circle is pi * the diameter. The diameter is the distance across the center of the circle from edge to edge. The radius of a circle is the distance from its center to its edge, and is always half of its diameter.

To figure out the diameter of a tire, and thus the distance it rolls in one turn, its easiest to first figure out the radius.

The radius of the tire is made up of three components - the tread depth, the sidewall height, and half the empty space where the wheel goes You can calculate the sidewall height and the empty space by knowing the tire size. The tread depth measurement you'd need to measure, and i'm not sure what depth is used when the tires sidewall height is calculated, so i don't know of the tread depth is additive or subtractive.

to calculate the sidewall height you just multiply the tire width (205mm) by the aspect ratio (55%). To get the distance of "empty space" you just cut the wheel diameter in half (we're talking about radius, remember). Since the wheel diameter is in inches and the section width of the tire is in millimeters, you need to do a unit version between millimeters and inches, depending on what units you want the answer to be in. I chose millimeters because the metric system is awesome and the US system is stupid

once the sidewall height has been added to the wheel radius, can you double that value to get the total diameter, and then multiple the diameter by pi to get the circumference, which is the approximate linear distance the tire will travel in one revolution along the ground.

what would make this more clear? pictures?

For the case above: 205/55R16

55% of 205 == .55 * 205 == 112.75mm

half of a 16" rim is 8". 1in = 25.4mm, so 8in is 203.2mm.

The radius of the tire is then 112.75 + 203.2 == 315.95mm

The diameter of the tire is 2x the radius, or 631.9mm.

The rolling distance (rd) of the tire is pi*d, where pi is however many digits you want to use, starting with 3.141592653

rd = 3.141592653 * 631.9 == 1985.17mm

for silly US units, that's a tire+wheel radius of 12.43", or a tire diameter of 24.8", and a rolling distance of 78 inches (6'6")

This level of "accuracy" is irrelevant, since inflation pressure will affect the amount of deformation during rolling. Additionally, speedometers have inaccuracy built in, usually between 3 and 10% @ 60mph. Finally, the wear of your tire will contirubte -- a new tire may have an 11/32nds tread depth, which is 8mm, while a worn tire could be be down to essentially 0mm. This constitutes a 16mm reduction in diameter, and thus a ~50mm reduction in rolling distance, which is just about 2 inches. A reduction 2 inches on a stated rolling distance of 72 inches constittues a 2% difference...just from tire wear.

bubba;180148 wrote:
it's not width it's circumference and how many rotations it makes.

WRONG! They will all make revolutions. It's diameter and how much area is covered per each revolution. A bigger diameter will cover more distance per revolution than a smaller diameter. Which means a taller tire will effectively lengthen your gear ratio and a shorter one does the opposite. That's just in the diameter part. Theres also the width which shouldn't have too much impact in this senario.

Tire sizes typically break down as follows. EXAMPLE P205/55R16 89H

P= Passenger vehicle

205= section width in millimeters (hardly ever exact but close)

55= Aspect ratio or profile. 55% of the width is the height of sidewall from bead area to outer edge of the tire. The lower the number, the lower the profile

R= radial construction. Most tires used by passenger cars and light trucks on the road today use a radial tire.

16= wheel diameter.

89H= load index and speed rating.

There's alot more to tires than just this. But with this little bit of info, you should be able to stumble your way through what you need.

For more info read this.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

Tire sizing tends to be a bit confusing to most. That is probably because the numbers aren't really accurate. A P205/50 VR15 tire size is translated as follows:

The "P" designates it as a passenger car tire. Many tires assuming this and do not show the letter. A "T" here would mean it is a temporary-use tire (your spare tire). This tire has a 205mm nominal sidewall-to-sidewall width, with a profile (or aspect ratio) of 50%, which is the height of the sidewall measured as a percentage of the tread width (102.5mm in this case). The "nominal" width means that this is an estimated number. The actual width will vary somewhat. The "V" is the speed rating of the tire, which is the maximum constant speed that the tire can handle. By exceeding this speed, you are at risk for tire failure due to heat build up and centripetal force. Below is a chart describing the speed ratings and their meanings.

bubba;180148 wrote:
it's not width it's circumference and how many rotations it makes.
thats exactly what i was thinking^^^^ can some explain whay exactly the width would matter if the tire is making the rotations at exactly the same amount as a tire that is slightly skinnier then it?

weshole;180145 wrote:
I used to use another program but now most of it is in my head and i can figure most out with just a calculator or pen n paper now.

what an awesome skill.

KA-T_240;180144 wrote:
Wes, it actually sounds like you used the tires size calc. off miataforums or whatever.

I used to use another program but now most of it is in my head and i can figure most out with just a calculator or pen n paper now.

Wes, it actually sounds like you used the tires size calc. off miataforums or whatever.