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Discussion Starter #1
What would be the best tire size for 17's on the focus?Also what would be close to the same height of the stock tires? and What is the differance between 215/45/17 and215/40/17's I dont know much about tire sizes?
 

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205-45 would be the closest to stock as you could get. Don't get the 215's becuase they are a higher profile rubber and provide less grip then the 205's. 45's are taller than the 40's. So if you are on 40's then you will get a little bit more torque on the launch due to the smaller tires, but your engine will work harder when going 80+mph, the speedo won't be noticably off but overtime your milage will read higher then it should due to the smaller tires. Just get some 205-45's and you will be set.
 

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Mr. Wizard
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focus said:
205-45 would be the closest to stock as you could get. Don't get the 215's becuase they are a higher profile rubber and provide less grip then the 205's. 45's are taller than the 40's. So if you are on 40's then you will get a little bit more torque on the launch due to the smaller tires, but your engine will work harder when going 80+mph, the speedo won't be noticably off but overtime your milage will read higher then it should due to the smaller tires. Just get some 205-45's and you will be set.
215 doesn't make it higher profile...215 is a wider section width, or contact patch, a 215 is a wider tire than a 205. The narrower the sidewall the stiffer the sidewall will be, the stiffer the sidewall, the less flex it will have and if you are gonna "drag" race with them, you will be more likely to spin the tires than to put traction on the ground. I don't know where you figured that the engine will work harder going 80+ mph? 205/40's don't offer very good protection for the rims and the sidewall is pretty small. 205/45 provides the closest match to stock if you have anything except for an SVT. 215/40 is a good compromise, but make sure that the tires provide a great enough load bearing capacity or their life span will be greatly reduced. 215/45 is a bit bigger, you get a wider tire, and a sidewall that is a good compromise between stiffness and protection for the rim. The other downside to 205/45's is availability, few tire makers have them, and when they do they are significantly more expensive.

Here are some numbers for you to work with...All compared with 195/60R15

205/40R17
Section width is 205mm
Tire can hold a 7-8 inch wide rim.
Overall diameter is 595.63mm
Sidewall is 81.788mm or about 3.22 inches
Circumference is 1871.2mm which will cause your speedometer to read slow, meaning you are going faster than it says. If your speedo said 60mph you'd be going about 62, a difference of 3.14%

205/45R17
Section width is 205mm
Tire can hold a 6.5-7.5 inch wide rim.
Overall diameter is 616.20mm
Sidewall is 92.202mm or 3.63inches
Circumference is 1935.7mm which will cause your speedometer to read too fast, meaning you are going slower than it says. If your speedo said 60mph you'd be going 59.87mph, a difference of 0.21%

215/40R17
Section width is 215mm
Tire can hold a 7-8.5 inch wide rim.
Overall diameter is 603.75mm
Sidewall is 85.852 or 3.38 inches.
Circumference is 1896.6mm which will cause your speedometer to read too slow, meaning you are going faster than it says. If your speedo said 60mph you'd be going 61.1mph, a difference of 1.82%

215/45R17
Section width is 215mm
Tire can hold a 7-8 inch wide rim.
Overall diameter is 625.09mm
Sidewall is 96.52mm or 3.8inches
Circumference is 1963.6mm which will cause your speedometer to read too fast, meaning you are going slower than it says. If your speedo said 60mph, you'd be going 59mph, a difference of 1.63%.
 

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I have 205/40/17 on my ride and whenever I pass those signs that show your MPH, I always seem to be going slower than what my speedo says......if my speedo says 46, I'm usually doing 44.
 

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Mr. Wizard
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BIGBADWORM said:
I have 205/40/17 on my ride and whenever I pass those signs that show your MPH, I always seem to be going slower than what my speedo says......if my speedo says 46, I'm usually doing 44.
Your speedo could be off, I think mine is, it says I'm going 40 but I'm usually going slower by radar. This is with the stock wheels on there. The numbers above are mathematically accurate. And assuming that your speedometer is correct, these numbers would be the result.
 

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Mr. Wizard
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overall traction should be similar between a 215/45 and a 215/40... 215/45 has a bigger sidewall so you get a little more flex which gives a bit better straight ahead traction without comprimising turning traction...These properties also have a great deal to do with what tires you get. Different compounds will react differently...I like a 215/45 they are readily available sizes from many tire makers and they offer protection for the rims as well as increased ride comfort. May not be important to you, it may be. I see alot of people with 215/45 but I'm from the Northeast and the roads can sometimes be a bit unforgiving.
 

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Discussion Starter #9
Tire Size Sidewall Diameter Circumference Rev/Mile Difference
205/50-16 4.0 in. 24.07 in. 75.62 in. 838 0.0%
205/45-17 3.6 in. 24.26 in. 76.23 in. 831 0.8%


acording to that site 205/45-17 are the closest to stock
 

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When I was looking for tires I was told that the 215's are a higher profile rubber and thus provide less grip even though there is more tire touching the road. As far as the 40's making the engine work harder at high speeds, that is correct. The total diameter for the 40's is smaller thus the engine as to turn the wheels more to cover the same distance and this will make your mileage read higher.
 

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Mr. Wizard
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drewzx3 said:
So I should go with 205/45's?
You should...they are the closest size to stock, and they will give you good performance characteristics...the reason that alot of people use other sizes is because there is very little selection in the 205/45 size range. And the selection that is available is typically very expensive as compared to other similar sizes.
 

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the first number in a tire size is the width of the tire in cm. the second number is the height of the tire in percent of its width, and the fourth is obviously the rim size.
 

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Mr. Wizard
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it's actually tire section width, slightly different than width, the number is in mm. The second number is a percent of the height to its width and the last is the rim size. The letter in front of the R (R for radius) just indicates the speed rating.
 

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Rotating mass becomes important on low horsepower cars. The statement that the motor will work harder with a larger tire is to, a degree, true. The further you move the mass (tire and rim) from the hub, more energy will be needed to turn them. To avoid this look for a light wheel AND tire. Too many people overlook the tires weight. There can be a pound difference between tires of the same size and even more as one increases tire size. If you're going to increase wheel diameter start looking at the combined weight not just that of the wheel.

Too many people spend a lot of money on light wheels and then mount bricks on them.
 

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I think you guys are spliting hairs here. Consider these facts:

1. There is often a slight size difference between tire makers on the same size tires.
2. As a tire wears the diameter becomes smaller.
3. New tires usually have between 7/32 - 11/32 tread depth.
4. 205-50-16 = 24.07 diameter
205-40-17 = 23.45 diameter
215-45-17 = 24.61 diameter

Consider these opinions:

1. The factory calibrates the speedo for a "worst case" ie: worn tires/small diameter.
2. My speedo on new factory tires seems off 1-2 mph.
3. Few cops will pull you over for 1-2-3 mph over.
4. Most cops will pull you over for 10-15-20 mph over.
5. Post here if you get busted for 10-15-20 over and you blame it on tires.
 

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BIGBADWORM said:
I have 205/40/17 on my ride and whenever I pass those signs that show your MPH, I always seem to be going slower than what my speedo says......if my speedo says 46, I'm usually doing 44.
I never thought those roadside radar signs were very accurate at all. My car is totally stock, and there is always about a 3-4 mph difference.
 

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Hi,

i know you are all domestic people here but if you can tolerate going to a Civic website there is a VERY nice tire size calculator there. You give it OEM stock info and info on a second tire/wheel combo and it will spit out all the important facs and draw you a visual of the tire diameter and width differences...

CLICK HERE for a link directly to the Tire Size Calculator

Enjoy!

Eric
 

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Mr. Wizard
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typically this information is found directly from the tire makers, some retailers have these specs on hand. You can figure out what you need for load bearing for your specific car from Ford directly and then you can either contact or check out www.tirerack.com or you can check any of the tire maker websites. You'll find the information there.
 
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