How to calculate tire sizes

[stealthee]

I am sure this has been covered in the past, and I think I have posted the formula for calculating tire diameter myself, but since the question "Will this fit?" comes up so often I figured this deserves its own dedicated thread.

Stock tire size on my 2007 Grand Prix GT is 225/55/17

225 is the section width in millimeters. To convert that to inches you would divide it by 25.4.
55 is percentage of diameter that is the sidewall height.
17 is, obviously, the rim diameter

The math formula to figure out the overall diameter would be the following:

((225/25.4) * .55) * 2) + 17 = 26.744

If you have Excel you can input the following formula

=SUM((((A1/25.4)*(B1*0.01)*2)+C1))

Place the tire size in A1, B1, and C1, and paste the fomula in D1

So 225 would be in A1, 55 in B1, 17 in C1 and the formula will automatically do the math for you. Since the sidwall height is based on the section width, if you go wider you will get taller unless you also change the sidewall ratio. A lot of people have trouble understaning this concept. You can use the formula to play with different widths and see how it affects the overall height.

Sure there are plenty of tire size calculators all over the internet, but if you have the formula saved on your desktop you can instantly see if your choice of wheel/tire combo will be too tall, or too short.

PS I wasn't sure where exactly a thread like this fits since we don't have a dedicated tire/wheel section, so if Alan or anyone thinks it needs to be moved, please do so.

[IPD]

Good post brian. It's a common question and very simple. Now let me add on a bit more:

Rim offset is the distance between the absolute centerline of the rim's width (cross section cut parallel to the face of the wheel) and the actual mounting point (where the wheel actually contacts the hub it's bolted onto). The distance between these two points is expressed in millimeters (mm) and is either positive or negative. A negative offset means that the spokes are further inboard than centerline. A positive offset means that the spokes are further outboard than centerline. In order have a good idea about fitment on a given set of wheels (excluding the infamous tt-caliper clearance), one must take offset into account to determine if the new tire/wheel combination will still clearance the strut/strut-mount, and/or if it will protrude further from the wheel arch.

There are offset calculators available online, however the math is fairly simple. Let's take my car for example. Stock wheels on my car were 16"x8" with (iirc) 40mm offset. Now a 0mm offset would mean that there would be 4" inboard and 4" outboard of the mounting point. 40mm is roughly equivalent to 1.57" (40/25.4), so in this particular case, there is 1.57" more rim inboard than if it had a 0mm offset. Thus there is 5.57" inboard and 2.43" outboard.

My new rims were 18"x10" 24mm offset. For this rim, 0mm offset would indicate 5" inboard, 5" outboard. 24/25.4 rounds to .94, so with a 24mm offset, there is 5.94 inches of rim inboard (.37 inches further inboard than stock). This leaves a remainder of 4.06" of rim outboard. Consequently, we can expect that the new combination will extend outwards 1.63" further than the old set.

Bear in mind that the tires selected for any given wheel combination can and will affect the clearance around the stock perches--as they extend around the rolling diameter, whereas coilovers simply sit inboard the rolling assembly.

[2fnloud]

How about:

tire calculator

and

offset

[R/T93]

Steve, this is 3/S land, no one searches for their own info, they need it spoon fed and laid out on a platter.

I use those links all the time

[IPD]

Steve, this is 3/S land, no one searches for their own info, they need it spoon fed and laid out on a platter.

I use those links all the time

regardless of how easy it is to use a calculator, the point remains that if you don't know how to do it yourself (i.e. understand the mechanics behind the concept) then you're simply a schmuck. you're probably the type of person who doesn't understand basic algebra & couldn't solve a word-problem to save his own life.

[TUFFTR]

regardless of how easy it is to use a calculator, the point remains that if you don't know how to do it yourself (i.e. understand the mechanics behind the concept) then you're simply a schmuck. you're probably the type of person who doesn't understand basic algebra & couldn't solve a word-problem to save his own life.

Sorry, so because I dont understand the fucking science behind everything in the universe and how it works I'm a shmuck? GTFO just use the friggin calculator.

And yes, Algebra can eat a dick. Give me a wiring diagram anyday.

[RealMcCoy]

How about:

tire calculator

and

offset

Both those links go to the tire size calculator...

[2fnloud]

Both those links go to the tire size calculator...

fixed.....I mean what are you talking about Willis, it is just fine

[R/T93]

regardless of how easy it is to use a calculator, the point remains that if you don't know how to do it yourself (i.e. understand the mechanics behind the concept) then you're simply a schmuck. you're probably the type of person who doesn't understand basic algebra & couldn't solve a word-problem to save his own life.

Since this is Brian's thread, I will refrain from calling you a retard, and its not the spokes, its the hub face.

Here is a better link that works pretty well, includes offset and wheel width, too, with comparison.
Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Thats cool you put it into excell, Brian.

[GTOJOE]

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Wheel Offset Calculator

Visual Tyre Size Calculator | Kouki Tech

It's nice to know how to do it but the calculator websites are much faster and easier. The only reason you would bother to do it manually is if your not in front of the PC. So in that case it's great info.