tire size.........wtf

[-Alero-]

i have a question and its bugging the hell out of me.

on a stock alero it can come in either 15's or 16's......my question is why are the tires 215/60/15 and 215/50/16

wouldnt that mean if your car came with the stock 15's and yo uchose 17's yo uwould have to use 50 series tires?

and if yoru car came stock with 16's and you chose 17's that would mean you would move to 45 series tires.

wtf is up with this. have i been running the wrong size tire all along? ( 225/45/17) mine came with 15's.

my geo has 175/65/14. that would mean if i had 17's on it i would have to have 50 series tires on it.

someone wanna explain this to me?

[CiscoPath]

I never understood this stuff either.

[-Alero-]

oh i understand the ratios. its just i dont understand why it skips 10 in series for 1 inch in rim. where it should only be 5 for every inch of rim.

[Redog]

15 inch Alero tires are 215/60/15 and 16's are 225/50/16

It's the plus size factor to keep the tire overall size the same. Typically you would go 10 mm on the width and series (sidewall) when you go up an inch and going down in size is the same way

[FantomForceCustoms02]

215/45/17 would be be the same overall wheel diameter as stock. The numbers are a percentage. I think it's 215 mm wide and the side wall is 45% of the 215. Something like that. 225/45/17 is close enough to keep your speedo correct. it would be off by about 1%

[Redog]

Originally Posted by -Alero-

my geo has 175/65/14. that would mean if i had 17's on it i would have to have 50 series tires on it.

205/50/17's

[-Alero-]

Originally Posted by Redog

205/50/17's

uhhhhh..........


why is there a 10 series difference between 15's and 16's why isnt it a 5 series difference. thats what im trying to find out.

why shouldnt we have 50 series tires with our 17's when our cars will come with 60 series tires on 15's!?

[Redog]

Some cars are different.

Alero's tire ratio's are "FUBAR" if you go with a 235/40/17 which in theory is the correct plus size.

[Cliff8928]

The Alero could come with either

215/60R15 25.2" dia (1999-2003)
205/65R15 25.5" dia (2004)
225/50R16 24.9" dia (1999-2004)

I used a 235/45R17 on mine.

[Alero2k4]

it is because the width is different. The '50' is an aspect of the 225, so because 225 is wider than 215, if it was a 215/xx/16 then it would need to be 55 but because it is 225/xx/16 it needs to be 50 if that makes sense....

[Cliff8928]

the actual math if that makes sense-

The tire size is calculated as the width (225) multiplied by the aspect ratio (.50), then doubled. Add that to the wheel size in mm (406.4). Divide that total number by 25.4 to get the overall tire diameter in inches. (24.9)

•225/50R16

2(225*.50)+(16*25.4)
225+406.4=631.4
631.4mm = 24.9"

[-Alero-]

hmmm mines an 02 and it came with the 15's and im running 225/45/17's right now so you think im off?

so your saying the lower the profile the wider the tire needs to be?

im wanting to go back to stock 15's but i dont have tires on the rims. what size do i need? 205/65 or 215/60

[AlbinoMonkeyRat]

it's because it's a game of percents.

example:
225/50/16 (stock)
225 = tread width (mm)
50 = % of the tread measure for the height of the sidewall (which then equals 112mm)
16 = inches diameter of the wheel (x25.6 for the measurement in mm = 409.6)

for overall diameter of the tire, add the rim to the sidewall measurement (x2) which is 634.6mm

up it to 17" rims.

17" = 435.2

634.6-435.2=199.4
199.4/2=99.7

99.7 is the ideal sidewall measurement.

keeping the 225mm tread measurement:
divide the tread measurement by 100 (the result is 1%)
then divide the ideal sidewall measurement by the 1% amount. This result is the actual % of the tread measurement that the ideal sidewall measurement is. in this particular case, 44.3111%. Since tire companies only make tires in increments of 5% series, you'd have to go with the 45 series.

thus the upsize would be 225/45/17.


If that made sense, I amaze myself, because I am drunk. If it doesn't make sense, then you know why. lol

edit: I just realized that I always mix up the conversion for inches to metric. use 25.4, not 25.6. sorry.

[Cliff8928]

Try this on for size...

http://www.miata.net/garage/tirecalc.html

[-Alero-]

god damn it my car is 2% too fast!?

my car probably doesnt have 69k on it then..........god damn it.

[jackal2000]

Originally Posted by -Alero-

god damn it my car is 2% too fast!?

my car probably doesnt have 69k on it then..........god damn it.

unless those miles were all highway then i dont think your odometer is too far off....

[-Alero-]

err well i first started out by moving from the stock rim/tirss to 205/40/r17 for like 30k miles and now i have 225/45/17 wich are off......apparently

[Redog]

Same size I have on my car, don't worry about it. I've driven past a few of those "You Speed" radar guns and the speedo is either dead on, or reads 1 mph faster