RXSeven1

what is the correct tire sizes for a 18x8, 18x8.5, and 18x9 rim?

swoope

iTrader: (5)

the outside diameter of the stock tire is 25.9"..

beers

RXSeven1

the part im confused about is the width. the correct size for our rim is 225/45 on a 18x8 rim now lets say i jump to a 18x9 rim. how do i know what width tire to choose with with geting one that is too wide and having too much sidewall flex or something to narrow for the rim.

Spinning Sushi

iTrader: (21)

Taj uses his stocks on his 18x9 rims with no problem

c41250n

iTrader: (5)

there is really no correct size! but there is a range of tire sizes that you can put on base on you wheel size!

G-ReX

If you want to keep the speedo accurate, use a calculator to maintain the outer diameter. 255/40-R18 is almost right on if you want to go wider and will be fine on a 9 inch wheel. You need to know where the offset will put the edge of the tire, too.
http://http://www.miata.net/garage/tirecalc.html

NgoRX8

iTrader: (1)

decide on one size wheel first. with offset. then it'd be easier to help.

staticlag

True!

There is the manufactuer size, but that is based on just what they wanted to put on the car at that time.

Generally I have found the optimal balance of tread width and response is:

8" = 235 is good

8.5 = 245 is good

9" = 255 is good

9.5 = 265 is good.

It all depends on what specific tire you run though, you could get 295s on a 9.5 and they would work great or a 245 on a 8 or 205 on an 8 for ultra response

But tire pressure also plays a factor in traction and response.

Spin9k

Think of it the other way around - pick a specific tire, and tire size you're interested in and then find the optimal wheel width for it. Now it's easy... as every manufacturer of tires has specific recommended wheel size for any tire size . . and then other wheel sizes that are 'OK', just not optimal.

It's pretty easy to find - go to the tire manuf of choice (or a site like tire rack, for example) and look at the spec for the tire you're interested in. They give a range of wheels sizes, but one size will be bolded or recommended - that's their determination of best wheel size for tire size.

How do they determine that? As mentioned, several wheel sizes will be shown in their chart to 'fit' on a given tire size, but for optimal tire footprint, wear, and sidewall support aka handling ... one size does the job for normal street driving. And it's different for different tires, different constructions, different tire applications. So don't go just by one manufacturer's suggestion and apply it to any other tire.

To narrow a wheel will buldge the tire center tread out, with implications for uneven tire wear over time or forcing you to use too little tire pressure to try to compensate. Too narrow wheel will allow to much flex in the sidewalls under lateral forces, giving a cushie ride, but subtracting crisp handling capabilities. Too wide wheel will cave the sidewall inward towards the tread giving less impact resistance than otherwise and again affecting ride and safety.

Simple eh? OK, then rethink some of that for aggressive or track driving as 'best width' will change depending on the application, but that's another story and a special case.