Ok. This may seem like a really dumb question but..how do I know what tires will fit a certain rim?

Let's say I want to fit some nice tires on some beautiful deep dish black bullits (18x9,18x10)?

[img]style_emoticons/<#EMO_DIR#>/icon_mrgreen.gif[/img]

Go to the tire manufacturer's websites, and they'll have specs on their tires which show the recommended rim widths.

The tire size is in three different units of measurement.
WIDTH in millimeters (MM):
For example, if you were to fit a tire onto an 18x9, then you would want a tread that is at least 9" wide, and better yet a little wider (to offer some rim protection against curb strikes. So, for 9", you ideally want 11" of rubber, so the first size is:
11 inches * 25.4 mm/inch = 280 mm
Tires come in increments of ten and ending in five, so you could get 275mm, 285mm, etc. Let's assume 275mm for this example (= 10.8 inches wide of rubber)
RIM DIAMETER in Inches:
In this case, it is a given = 18 inches.
ASPECT RATIO in Percent:
This is the percent of the width that is the rubber height. The stock GT wheel has an overall diameter of 27.2", so you want to balance out things to get the wheel to the right diameter. To calculate the desired aspect ratio, it is:
27.2 inches = 18 inches + ((275mm) / (25.4 mm/inch)) * aspect ratio * 2
aspect ratio = 42%
But, rubber comes only in 5% increments, so let's choose 40%.
FINAL ERROR:
So all of this translates into a 275/40R18 tire, which will have an outside diameter of:
18 inches + ((275 mm) / (25.4 mm/in)) * 40% * 2 = 26.7 inches.
At 60 MPH indicated on your speedometer, you will actually be traveling at:
60 MPH * 26.7"/27.2" = 59 MPH. (for the 18x9 rim with a 275/40R18 tire, and this speed error only applies if placed on the rear, as that is where the speed pickup resides)

If the 18x10 is chosen for the rear, then I suggest one of these two options:
315 mm = 12.4 inches wide
315/35R18 will give you an outside diameter of 26.7”, same as above.

305 mm = 12.0 inches wide
305/40R18 will give you an outside diameter of 27.6”.

<div class='quotetop'>QUOTE(Gearhead @ February 9, 2006, 9:43 PM) Quoted post</div><div class='quotemain'>
The tire size is in three different units of measurement.
WIDTH in millimeters (MM):
For example, if you were to fit a tire onto an 18x9, then you would want a tread that is at least 9" wide, and better yet a little wider (to offer some rim protection against curb strikes. So, for 9", you ideally want 11" of rubber, so the first size is:
11 inches * 25.4 mm/inch = 280 mm
Tires come in increments of ten and ending in five, so you could get 275mm, 285mm, etc. Let's assume 275mm for this example (= 10.8 inches wide of rubber)
RIM DIAMETER in Inches:
In this case, it is a given = 18 inches.
ASPECT RATIO in Percent:
This is the percent of the width that is the rubber height. The stock GT wheel has an overall diameter of 27.2", so you want to balance out things to get the wheel to the right diameter. To calculate the desired aspect ratio, it is:
27.2 inches = 18 inches + ((275mm) / (25.4 mm/inch)) * aspect ratio * 2
aspect ratio = 42%
But, rubber comes only in 5% increments, so let's choose 40%.
FINAL ERROR:
So all of this translates into a 275/40R18 tire, which will have an outside diameter of:
18 inches + ((275 mm) / (25.4 mm/in)) * 40% * 2 = 26.7 inches.
At 60 MPH indicated on your speedometer, you will actually be traveling at:
60 MPH * 26.7"/27.2" = 59 MPH. (for the 18x9 rim with a 275/40R18 tire, and this speed error only applies if placed on the rear, as that is where the speed pickup resides)

If the 18x10 is chosen for the rear, then I suggest one of these two options:
315 mm = 12.4 inches wide
315/35R18 will give you an outside diameter of 26.7”, same as above.

305 mm = 12.0 inches wide
305/40R18 will give you an outside diameter of 27.6”.
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And now we know why he's called Gearhead! [img]style_emoticons/<#EMO_DIR#>/bowdown.gif[/img] [img]style_emoticons/<#EMO_DIR#>/nixon.gif[/img]

315 or 305 in the rear? Has anyone done that yet?

I believe the "Boy Racer" had 315s in the back. And I know the Steeda had 305s.

I went to my car dealership yesterday, and they had an '06 yellow convertible that they're customizing. It's got Flowmaster mufflers, 18" Ford chrome bullitts, and Toyo tires. The tire sizes were 255/45 in front, and 285/40 in the back.

They claimed those were the biggest tires you could put on the 18" Ford rims. Why can't the 285/40-18 tires go in the front? wheel well clearance problems?

More than likely it's because the ford 18's are 8.5" wide. A 285 is 11 1/4" wide. That's a lot of unsupported rubber on your steering tires.

<div class='quotetop'>QUOTE(wjones14 @ February 10, 2006, 9:23 AM) Quoted post</div><div class='quotemain'>
I went to my car dealership yesterday, and they had an '06 yellow convertible that they're customizing. It's got Flowmaster mufflers, 18" Ford chrome bullitts, and Toyo tires. The tire sizes were 255/45 in front, and 285/40 in the back.

They claimed those were the biggest tires you could put on the 18" Ford rims. Why can't the 285/40-18 tires go in the front? wheel well clearance problems?
[/b][/quote]

What brand of tires?

The Les Schwab Tire Dealer from whom I purchased my tires & wheels has successfully installed 275 mm up front, and 315 mm in the back. He said that while he thought he might even get a little wider in the rear, he felt that 275 mm up front was about it for wheel well clearance.