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Old 10-22-2012, 10:42 AM   #1
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Increasing Tire Width

I have a '12 focus SEL w/ 215/50/17 contis on it.

I don't like the tires and want to change them out and increase the width of the tire to 225/50/17

I've uped the width before on other cars (while retaining the same aspect ration and wheel size) w/o any problems that i could see but i've been told that if i increase the width i have to reduce the aspect ration to 45. I don't see how increasing the width by 10mm will negatively affect the car. It's my understanding that there is a +/- 2% rule i have to follow but if only the width increases 10mm does the aspect ration change too?

Any info you can give will be appreciated..


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Old 10-22-2012, 11:00 AM   #2
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the rule of thumb then changes sizes is 10/5 increase width by 10mm decrease aspect by 5%, and when decreasing width by 10mm increase aspect by 5%

the way tires sizes work is that the aspect ratio is the percentage of the tire width.

example 215/50 has a sidewall height 107.5mm
and a 225/50 is 112.5mm
by using the 10/5 rule the proper upsize is 225/45 which gives a sidewall height of 101.25

now a 17 inch rim has a diameter of 431.8mm add the sidewall heights of the numbers above by 2x (top and bottom of the tire. you get

215/50= 646.8mm tall
225/50= 656.8mm tall
225/50= 634.3mm tall

So 225/50 is 10mm taller than stock, 225/45 is 12.5mm shorter than stock. given this information, even tho 10 mm taller might not seam like much, in the world of suspension geometry 10mm can be a big issue. will you be able to run 225/50 most likely, but the only issue is see is possible rubbing because of a larger size. where going 225/45 you will actually minimize the risk or rubbing with an overall smaller diameter.

No all my math is pretty general, there are more factors that play as well, such as tread depth, and where the individual tire manufacturer takes their measurements from (there is no standard on where tire measurements are taken from each manufacturer is allowed to make its own measuring criteria) but what i put together above is pretty general and show how tire sizes work and show what happens when you do change sizes.

If it were me I would go for 225/45
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Old 10-22-2012, 12:26 PM   #3
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Let's try to make this a little simpler if possible and discuss a couple of other impacts that are pertinent. First the tire sizes in inches:

225/45-17 = 25.0" in diameter
215/50-17 = 25.5"
225/50-17 = 25.9"

As you can see all the two optional tires are within a half inch of the stock 215/50-17. That should not create any major drive-ability issues. But there are more subtle issues. The bigger tire will have more rotating mass. This will have impacts on acceleration (more power to get them turning and a slight mpg hit as well) and more energy to stop them (possible modest increases in stopping distances). The smaller tire, on the other hand will slightly modify the final gear ratio. This could improve acceleration but the motor will be turning higher RPM's when cruising. This could also impact mpg. It could be argued that the higher RPM could cause increased engine wear but that would require lots of highway miles.

The increased width of the tires will also have some modest impacts on adhesion which could also impact gas mileage but may have offsetting benefits in handling from increased lateral traction.

So in summary it kind of looks like a wash. From a performance standpoint its unlikely you be gaining any significant performance enhancements. From a "looks" standpoint, I doubt anyone could visually tell the difference in sizes. All that said you could go with any of the sizes mentioned. Naturally the larger sizes may create fitment issues if you're lowered or intend to lower the car. I suggest a test fit if you want to go in this direction.
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Old 10-22-2012, 01:10 PM   #4
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Quote:
Originally Posted by Geezer View Post
Let's try to make this a little simpler if possible and discuss a couple of other impacts that are pertinent. First the tire sizes in inches:

225/45-17 = 25.0" in diameter
215/50-17 = 25.5"
225/50-17 = 25.9"

As you can see all the two optional tires are within a half inch of the stock 215/50-17. That should not create any major drive-ability issues. But there are more subtle issues. The bigger tire will have more rotating mass. This will have impacts on acceleration (more power to get them turning and a slight mpg hit as well) and more energy to stop them (possible modest increases in stopping distances). The smaller tire, on the other hand will slightly modify the final gear ratio. This could improve acceleration but the motor will be turning higher RPM's when cruising. This could also impact mpg. It could be argued that the higher RPM could cause increased engine wear but that would require lots of highway miles.

The increased width of the tires will also have some modest impacts on adhesion which could also impact gas mileage but may have offsetting benefits in handling from increased lateral traction.

So in summary it kind of looks like a wash. From a performance standpoint its unlikely you be gaining any significant performance enhancements. From a "looks" standpoint, I doubt anyone could visually tell the difference in sizes. All that said you could go with any of the sizes mentioned. Naturally the larger sizes may create fitment issues if you're lowered or intend to lower the car. I suggest a test fit if you want to go in this direction.

I appreciate all the info presented to me about my question... My initial reason for increasing width was both for looks and perhaps a bit more adhesion due to the wider tire but i can see now that this isn't the issue so much.
Perhaps i need to stick with the original tire size but look for a tire with a much stiffer sidewall? Opinions?

On another tact, any recommendations for increasing handling by intalling 'better' shocks and struts than the OEM ones?

This all came about after driving my sister's Ti this weekend and i was shocked at the diff in handling from my SEL (which i had been previously satisfied with). So now looking for ways to , even marginally, better the handling characteristics of the car. thanks all.
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Old 10-22-2012, 03:13 PM   #5
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Does she have the 18s on here car and the handling package
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Old 10-23-2012, 08:26 AM   #6
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Does she have the 18s on here car and the handling package
yes she does have the 18s so also the 'sport handling' pkg.

My SEL, of course doesn't have the sport pkg but i was hoping that upgrading the tires to a bit wider (or w/ a 45 vs.50 aspect ratio, or 50 series w/ a stiffer sidewall) and better shocks/struts would help out in the handling dept a bit.

opinions?
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Old 10-23-2012, 10:15 AM   #7
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Yes, tires and upgraded suspension components certainly could help. You don't necessarily have to go to 18 inch wheels and tires to do so. There's plenty of performance tires in smaller diameters available. Right now there's just not a lot of suspension options for the 3rd gen cars. Apparently Ford is making the sport package available to the aftermarket. I believe I've seen numbers like $600.00 being thrown around here for the package. If currently available I'm sure that Tousley Ford will offer it at a competitive price. I'm sure we'll being seeing more options and upgrades for the 3rd gen cars in the near future from other manufacturers.
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