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Old 09-06-2009, 12:00 AM   #1
PleiadesSTi
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Default Help me understand

Why everyone is so apt to go from 17in to 18in etc sized rims just to fill in the fender well? Don't you realize that your speedometer is off and your gearing ratios have changed?

I'm confused by this trend on this forum.

I'm all for reducing fender well gap, but not at the price of ****ing my mileage and gearing ratios up.

Unless you can relfash for larger diameter tires/rims?

I dunno. I rather just lower the darn thing on coilovers and not mess around with larger diameter tires/rims.

If this logic is wrong please guide in the right direction.
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Old 09-06-2009, 12:32 AM   #2
decimal
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not sure where you're seeing people go to larger tires to get rid of wheel gap. When you plus size the wheel, you downsize the sidewall to match, so the overall diameter doesnt change.
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Old 09-06-2009, 07:39 AM   #3
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^ What he said ^ Going from a 225/45/17, for example, to a XXX/40/18 or 35/18 will cut down on that inch you just gained.
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Old 09-06-2009, 09:22 AM   #4
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Heres how your speedo will be affected at 60mph... starting with 22/45/17

If you go to...

235/40/18 -- actual speed will be 61 mph
245/40/18 -- actual speed will be 61.8 mph
245/35/18 -- actual speed will be 59.5 mph

As you can see, this is hardly any difference at all.
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Old 09-06-2009, 09:38 AM   #5
Charlie-III
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As mentioned, correctly doing a "+1" or "+2" tire/wheel combo retains the gearing and speedo accuracy.

Granted, the rims are usually heavier with the greatest mass further out which is bad for acceleration.

But hey, if that's what they want to do, then fine by me.
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Old 09-06-2009, 09:55 AM   #6
PleiadesSTi
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Quote:
Originally Posted by Charlie-III View Post
As mentioned, correctly doing a "+1" or "+2" tire/wheel combo retains the gearing and speedo accuracy.

Granted, the rims are usually heavier with the greatest mass further out which is bad for acceleration.

But hey, if that's what they want to do, then fine by me.
No, I'm into making my wrx faster not slower.

i'll stick with suspension mods rather than getting huge rims.
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Old 09-06-2009, 12:31 PM   #7
LorenzoS
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Since I put the 305/25-20's on my wagon, my speedometer is only off 4% but you should see how da honeys be checkin out my ride.
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Old 09-06-2009, 10:05 PM   #8
PleiadesSTi
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Even a little change bothers me. Your mileage will not be correct, and with a warranty it bothers me even more.
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Old 09-06-2009, 11:00 PM   #9
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^ haha you're such a newb it's not even funny

do some research man. You have no idea what you are talking about. I usually don't bash on newbs, but this thread is pointless.

18's aren't huge "rims"...you find what height in inches your factory tires are and try to keep it really close to those specs. Literally every tire has their specs posted somewhere (usually on the manufacturers or dealers website) if you look.

Your logic is completely wrong.

Here's some generally based tire height info - http://www.jekylhyderacing.com/HeightofTires.htm

some people actually go down in tire height slightly to gain a better gearing ratio. It's pretty common for WRX (5speed) owners to go to a 235/40/17 sized tire. Your speedo will be off hardly a noticeable amount, but you will notice the difference in (better) acceleration.

tires don't affect your warranty. You are worried about mileage so much? That's what bothers me. You aren't going to notice much of a difference at all with any size close to factory specs if you are driving to save mileage anyways. I could understand if you put some 40lb. chrome 20's on, but we aren't talking about that. You can get lightweight 18's that would be better for performance than your stock 17's.
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Old 09-06-2009, 11:41 PM   #10
PleiadesSTi
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Quote:
Originally Posted by grippy monkey View Post
^ haha you're such a newb it's not even funny

do some research man. You have no idea what you are talking about. I usually don't bash on newbs, but this thread is pointless.

18's aren't huge "rims"...you find what height in inches your factory tires are and try to keep it really close to those specs. Literally every tire has their specs posted somewhere (usually on the manufacturers or dealers website) if you look.

Your logic is completely wrong.

Here's some generally based tire height info - http://www.jekylhyderacing.com/HeightofTires.htm

some people actually go down in tire height slightly to gain a better gearing ratio. It's pretty common for WRX (5speed) owners to go to a 235/40/17 sized tire. Your speedo will be off hardly a noticeable amount, but you will notice the difference in (better) acceleration.

tires don't affect your warranty. You are worried about mileage so much? That's what bothers me. You aren't going to notice much of a difference at all with any size close to factory specs if you are driving to save mileage anyways. I could understand if you put some 40lb. chrome 20's on, but we aren't talking about that. You can get lightweight 18's that would be better for performance than your stock 17's.
You don't have to get all mean about it. Unlike a lot of people I'm here to learn. Thats why I posted this to LEARN!

I think I know what you are saying. You can keep the same diameter in an 18in tire as you can a 17in tire, it just depends on rim width, sidewall height wanted etc. Correct?
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Old 09-07-2009, 12:36 AM   #11
Uncle Scotty
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OK......here it is simple

If you want a bit more tire than stock---we'll use a 17x7 wheel and 215/45 tire as the stock size example......
In order to go to a larger tire and maintain the stock tire OD...and NOT alter the final drive ratio, which affects the spedometer.......
You must change the 'aspect ratio'--the '45' in the above tire size.....that is lower it to(for this example) '40' and increase the tire width (in mm)---the 215 as above.....to 245.
this makes the tire sidewall shorter in proportion to the wider width, while maintaining the OD
This is all approximate, and will vary by a few percent for many possible reasons, but for the sake of this experiment, we will ignore this minor(and irrelevant to most of us) error.

So now we have a wider tire, and need a wider wheel in order to maintain symmetry of the tire when mounted(some autox users ignore this for their specific, non-street, use and their usage does not apply here).

Tire manufacturers have a 'recommended wheel width' range for their tires......and our 'NEW' size of 245/40 in a RE-11 has an 8-9.5" recommended wheel width range, so the new wheel would be in that width range to be 'correct'

For an 18" wheel, the tire would have to be 225/40 or 255/35 with the wheel to fit those widths
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Old 09-07-2009, 04:26 PM   #12
PleiadesSTi
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Quote:
Originally Posted by Uncle Scotty View Post
OK......here it is simple

If you want a bit more tire than stock---we'll use a 17x7 wheel and 215/45 tire as the stock size example......
In order to go to a larger tire and maintain the stock tire OD...and NOT alter the final drive ratio, which affects the spedometer.......
You must change the 'aspect ratio'--the '45' in the above tire size.....that is lower it to(for this example) '40' and increase the tire width (in mm)---the 215 as above.....to 245.
this makes the tire sidewall shorter in proportion to the wider width, while maintaining the OD
This is all approximate, and will vary by a few percent for many possible reasons, but for the sake of this experiment, we will ignore this minor(and irrelevant to most of us) error.

So now we have a wider tire, and need a wider wheel in order to maintain symmetry of the tire when mounted(some autox users ignore this for their specific, non-street, use and their usage does not apply here).

Tire manufacturers have a 'recommended wheel width' range for their tires......and our 'NEW' size of 245/40 in a RE-11 has an 8-9.5" recommended wheel width range, so the new wheel would be in that width range to be 'correct'

For an 18" wheel, the tire would have to be 225/40 or 255/35 with the wheel to fit those widths
Ok, it makes sense now. Thank you!!
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