Homer

iTrader: (1)

tire/wheel sizes I just don't get. Wheel sizes make sense. 15 x 9 is a 15" diameter by 9" width. I know what the numbers on tire sizes mean, but I can't tell if they would match a particular rim or not. 185/70/r14 is a tire for a 14 inch rim with a 70 aspect ratio and 185 circumference (i think). but what width is that? would that be a 14 x 6 or a 14 x 7? Can someone make sense of tire/rim sizes for me?

RB83L69

185/70-14:

• 185mm = section width (the widest part of the tire, usually somewhere in the middle of the sidewall)
• 70 = aspect ratio; the sidewall height is 70% of the section width
• 14 = wheel diameter

Generally, you want a tire with a section width that's about 1 - 1˝" wider than the wheel. That's about 25-40mm. If the tire is wider than that it will balloon out from the wheel and the tread will be distorted; if it's too narrow it will look like the ricers with the ridiculous low-profile tires they put on 20" wheels.

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"So many Mustangs, so little time..."
ICON Motorsports

Dan91Z

(S * A * 2) / 25.4 + R = T

S= Section Wdith (eg. 245)
A= Aspect Ratio (eg. 50)
R= Rim Diameter (eg. 16)
T= Tire Diameter (Which we are solving for)

So, a P245/50R16 would be 25.6 Inches tall,
and a P275/40R17 would be 25.7 Inches tall.

To find the width in inches,

width in inches = section height / 25.4

so, a 185 tire would be 185/25.4 which comes out to 7.283 inches wide.


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My `91 Z28 Convertible

Northern Kentucky Fbodies

Damon

That's good info on determining tire size, above.

I'll jsut throw in there's an old "rule of thumb" that says the wheel should be at least 75% as wide as the tire's WIDTH to keep a good contact patch on the road. Putting a too-wide tire on a too-narrow wheel will result in the tread balooning out at the middle and wearing the middle of the tread out quickly. Not to mention it won't grip the road very well.

JP84Z430HP

Wow, this stuff would make a good tech article!

I always thought that the width spec was for the tread width, but the section width seems more realistic.

I keep learning things here! It's starting to hurt my brain!

AlkyIROC

Another thing to consider is where the tire is measured. A P245/50R16 tire doesn't have a 245 tread but is measured at the sidewall bulge. All DOT tires are measured this way.

MT ET Street tires are DOT tires. A 26 x 10.5 x 15 ET Street has about the same tread width as a 26 x 9 x 15 ET Drag. Racing slicks are measured by the actual tread width.

Taller tires can give you more rubber on the ground if you can't fit wider ones on since the circumfrence is larger. That's why Stock Eliminator cars are limited to a 9" wide tread with a maximum height of 30". Very few run a tire smaller than 29" since they are giving up valuable traction. When using such a tall tire, gears such as 4.56 become reasonable. The tall tire gives more rubber on the ground and the deeper gear gives torque multiplication to get the car moving.

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87 IROC-Z Pro ET Bracket Race Car
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Best results before the 383 blew up
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RICHRAD

hey Dan91z, I've added and added and added and it doesn't figure. Is something missing from the tire diameter formula?? I'm trying to calculate the 245/50/15s I've got coming from a seller. Or is it the width I need to look at to determine If they will be allright on 15 x 7s?

RB83L69

That is too wide to fit properly on a 7" wheel. 235/60 is about as wide as you can go. 245s will look like balloons, as described above, and the edges of the tread will never touch the paevement so you'll have short tread life and poor traction compared to using a slightly narrower tire.

A 7" wheel is about 175mm wide, so you're putting a tire on that's 70mm wider, or close to 3" (1 in = 25.4 mm) So your sidewall will end up 1˝" farther out than the rim. Poor fit to say the least.

Some people will tell you that it works, that they've done it without problems even, but it's still not right.

245 * .50 * 2 / 25.4 = about 9.6"; 9.6 + 16 = 25.6". In your case, using the same width and aspect ratio, you will have an outer diameter of about 24.6". Your original 215/65-15s were about 26.1" IIRC. So you're putting a much smaller diameter tire on there.

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"So many Mustangs, so little time..."
ICON Motorsports

RICHRAD

well' I've been to the tire mfg. website since and it seems like I've shoehorned my way in to that one. They claim my dunlops go on min 7" and max 8.5" rims .So my contact patch should be just fine.