Need new rubber for my lowered (Z springs) 03 sedan and was wondering what size tire to go with between a 245/40/18 or a 225/45/18 and why.

Thanks

BTW they will be Falken's 452

 

Does anybody have a pic of the 245/40/18 on a l03 owered sedan.

Thanks

 

I'll let you do the leg work, but use this to enter your OEM tire size and then your new tire.

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

Make sure your rolling distance is the same (so your speedo and odometer will still function correctly) *ie keep the circles overlapping with new/old set ups. Then upgrade as much as you want but make sure to keep the width appropriate for the rim. Too narrow and you're stretching, and too wide, you're gonna flop about. Try to find a size that mimics OEM rolling diameter and plus size your width +10 or +20, any wider and you will probably get a tire to wide for your rim and it will detract from handling.

EDIT: I ran it just for fun. Given a 225/45/18, a 245/40/18 upgrade would give a nearly identical tire size and should work just fine. It will read 1% too fast, ie at 60mph on your speedo, you will actually be going 59.4, which is good enough I think. So if you have that 225 setup, upgrading to a 245/40 is your best bet, a 235 wont get you the same size.
If you're looking to upgrade, you got the right size.

 

OEM is 235/45/18 which is 668mm, I am running 245/45/18 (677mm)
which places the car too high for my standards, so was wondering
between the 225/45/18 (659mm) or 245/40/18 (653), I'm leaning
towards 225 as it would keep it closer to stock..Right now I'm running
my winter set up 215/60/16 (664mm) and everyone say that car is lowered
but they never say it with my summer set up..

GBoy

 

With a 235/45/18, your closest matches for tire size are now

225/45/18, you're losing an extra 10mm of contact patch and you'll be running 1.3% a fast speedometer.

otherwise

255/40/18, you're gaining an extra 20mm of contact and you'll be running a 1.1% fast speedometer.

Either of these sizes would fit just fine in terms of rolling diameter. As for tire width, make sure your wheel is capable of taking a 255, chances are it can, but make sure, but know that you will have a bit extra side wall kinda rolling around.

You don't ever want to choose the 225 because you are essentially losing tire width and also messing up your speedo for it. No reason. Always upgrade, never go down in tire size unless its for winter tires.
If I were you, I'd stay with the stock size in the new tire or if you want, go +2 with the 255/40 setup which should fit just fine, but be sure. It may handle worse despite the added tire width. Again OEM size in the new tire is what I'd recommend if you aren't sure about the 255.

 

OEM is 235/45/18 which is 668mm, I am running 245/45/18 (677mm)
which places the car too high for my standards, so was wondering
between the 225/45/18 (659mm) or 245/40/18 (653), I'm leaning
towards 225 as it would keep it closer to stock..Right now I'm running
my winter set up 215/60/16 (664mm) and everyone say that car is lowered
but they never say it with my summer set up..

GBoy

 

oops sorry for resending my response, trying to do many things at once
255 seems quite wide, my 245/45/18 seems slow to move off from a start
so definately want a smaller diameter than 245/45 (677), I will check
255, thanks..

GBOy

 

Yeah 255 might be a little gummy for your wheels.
I say stick with OEM, your 245 set up is messing up your speedo and odo by 2.3%, definetly don't stick with that size. Stay with 235/45/18...its OEM and recommended, you should never go down from there, thats just giving away performance.

 

Spoke with tirerack, and they said they could not guarantee against rubbing
with a 255, they said I can use a 245/40/18 or 225/45/18 so back
to my original question, which one to choose and why..

GBoy

 

If you want to keep your 18's and have tires with a shorter sidewall your speedo will be off, there is no way around that.

If you can fit a 245 I would not go down from there as it will hurt performance. Using a 215/55/17 which was my stock tire as a reference I come up with these numbers. Your current size of 245/45/18 is 1.4% larger than the stock size so your speedo is 1.4% slow. If you go down to a 245/40/18 your speedo will be 2.3% fast. So when your speedo says 60mph you will be going 58.6.

Now these numbers are NOT exact since all tires vary a little from company to company but this will get you close. IMO 2.3% slow is not a big deal if it makes you happy with your car.

 

I am a bit confused with the calculator as it does not take into account the rim width, which if I am looking at this correct a stretched 275/30/20 on a 10.5" rim would be different in height then a 275/30/20 on a 9.5" rim but still be doable. Is there a calculator you can take the width of the rim into account?

 

The calculator does have a rim size factor built in. You choose it as the third variable, right next to the size and profile...
http://www.miata.net/garage/tirecalc.html
check again, hope this helps

 

???

 

I don't see anything that gives me true width of the rim.

For instance, I have OEM 18" rims, width 8", with tires 225/45/18.
If I buy 20" rims that are 9" wide what will come closer to my current radius, 245/35/20 or 255/30/20?

Likewise for the rear I have 18" rims, width 8.5"(I think?) they are 04 OEM coupe rims. Tires are 245/45/18.
If I put 20" rims that are 10.5" wide what will come closer to my current radius, 285/30/20 or 275/35/20?

See the tire calculator doesn't take into effect stretching tires on rims which many do thus a 275/35 and a 285/30 can both go on the same rim width of 10.5".

Anyone know what looks better drives better ect..?

 

Shoot, haha, I'm sorry. I didn't read your post closely enough, I just assumed you had said diameter, and I was in a seminar so I replied quickly.
But you are right, there is no wheel width variable, that is something you have to figure out on your own. The manufacturer should recommend some specs about narrowest and widest tires for each wheel. Narrow is gonna stretch, wide is gonna wobble on the sidewall...I would look around to see what setups other people have used and judge by that means. There are some consensus measurements that seem to be used for certain wheel and width combinations, I would try to find those out and go from there.