the aspect ratio is the SIDEWALL. this is the sidewall

if you want skinny sidewalls then you get big rims, because the overall wheel diameter needs to stay constant. so if you get a 10 inch rim then you would have to have a huge sidewall. but with the ones you said, 17 in the front and 18 in the back itll look like this

so if you want 315 in the back you need a 315/30-18 tire size.

 

I know what the sidewall itself is but thanks for the example man. I understand it now.

 

315-45-17 fits just fine

 

did you have your speedometer re-calibrated?

You are running a larger aspect ratio than I with my 275 40 17's and thus your tire is taller.

It's also like going with a taller gear ratio

your 45 ratio probably should have been more like a 35 to stay in the 25 1/2 inch height range


it's not just about width which it seems many are concerned about in this thread. You just can't go and increase the tire diameter by 3-5 inches.

Not to pick nits, but to state that 315 x 45 x 17's fit just fine may be true that they physically fit, but are they sized correctly and did you re-gear to compensate for what you lost and also recalibrate your speedometer?

 

right now i'm running 245-50-16 all around, they were on the car when i got it and are almost brand new... Ive been reading all these threads and am a little confused, what size should i go with to upgrade the rears? I'm running Kumho Ecsta's.

 

Re-gear? Are you saying he should re-gear just because of different size tires? That's a little much isn't it?

 

if power is an issue with you, then instead of re-gearing, just choose a smaller side wall/aspect ratio. you will lose a little power if the diameter of the wheel/tire is bigger than stock. but if its just for looks, then re-gearing isnt necessary but at least recalibrating the speedo is. i got busted once cus my speedo is off about 5mph so technically, i was speeding.

 

if you are running 25 1/2 inch high tires

and then switch to 3 inch higher tires which is about what 315 x 45's are, then you not only affect your speedometer but it's like changing gear size

so, if you want to remain equal to before, then yes

or, you ca simply match your apsect ratio to where it should be which is an easier thing to do.

some in this thread, for some reason have taken facts and tossed it aside as unimportant and focuses solely on width.

 

depends, first, 245/50-16 is the stock size. if you want to go wider then you would have to buy new rims that match the tire widt. you want to upgrade to. I, personally, wanted widest possible so I went with 18"x10.5"
with 295/35-18 tires. switching to that big of a wheel has some upsides and downsides, you get amazing traction and it looks awesome but it decreases your gas mileage a bit. if you want something in between go with 275/40-17 or something around that size which would need a 17"x9.5" rim.

 

you don't have to get bigger rims you can put a 12" tire on an 8.5" rim. the tire will be a little taller and stick out from the rim but you will not curb a wheele doing this.