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All you could EVER want to know about Tire Sizes (then some)

All you could EVER want to know about Tire Sizes (then some)

Postby theprodigy79 » Thu Aug 25, 2005 7:37 am


Just to make everything clear for those who don't understand about the tire size ratings (I'm not pointing anyone out here, this is for anyone who is interested or doesn't understand... it's just a good knowledge to have)... I'll take a generic tire size (my own current tire)...

Let's use 225/45ZR18

225 is the tire width in mm. 45 is the side wall aspect ratio in %. Z is the tire rating (not always listed in that particular spot, but sometimes placed there). R represents if the tire is a Radial or not (steel mesh belts wrapped around the tire). 18 is the wheel size the tire will fit on in inches (good ol American contrast with mm and inches on the product)

To find the size of the side wall in mm, multiply the tire width by the side wall aspect ratio (as a decimal, so for 45% it would be .45)

In this case, the side walls are 101.25mm (225x.45)

If you are sticking with one rim size, that's pretty much all you need to know... HOWEVER, many of you are upgrading rim sizes (if you haven't already... So it gets just a tad more complicated (not much though).

Besides possible clearance issues, the only other issue with changing your tire diameter is throwing off your speedometer (which you can easily have recalibrated at a place such as Precision Tune or the like). There are generally no other detrimental effects of switching tire sizes (keep in mind, smaller diameter tires will increase your accelleration and drop your top end speed, while larger diameters will lower your accelleration and increase your top end speed).

If you don't want to mess with recalibrating your speedometer, however, you need to keep your tire diameter within around 3% of it's original size (just a rule of thumb)... How do we figure out the overall diameter of the tire?

Let's go back to our 225/45R18s, shall we?

Okay, so we already determined the side walls are 101.25mm... We now need to include the overall diameter of the rim... But Americans are pretty stupid, and the rim is in INCHES!!! What now?!? Simple... One inch is equal to 25.4mm. So lets find this diameter! Also, we must remember that we have TWO side walls across one axis!!! Our general formula is as follows:

Tire width (mm) = W, Side wall aspect ratio (decimal) = S, Rim size (inches) = R

W x S x 2 + R x 25.4 = Overall Diameter (D)

To clean that up a bit (yay, algebra!):

2(W x S) + 25.4(R) = D

So our setup for this tire is:

2(225 x .45) + 25.4(18 ) = ???

Now, if you remember simple 4th grade mathematics, you will remember that multiplication comes before addition (unless brackets are involved!) So our OVERALL tire diameter here would be:

659.7mm TA DA!

18" turns out to be 457.2mm, so you add that to 2 of our 101.25mm side walls (202.5mm total) and BAM! (Emeril)

So you have 17" rims and you wanna upgrade to 18"??

My Solara Gen2 came with 17" wrapped with 215/55VR17s... I wanted to upgrade to 18"s... How?

Okay, I want to find the overall diameter of the factory tires:

(215 x 55%)(2) + 17"x25.4mm = 118.25mm x 2 + 431.8mm = 668.3mm

So we need to find a tire within around 3% of 668.3mm (I prefer somewhere in the 1%s, but that's just me)... So we can have + or - 20.05mm... meaning anywhere in the range of 648.25mm to 688.35mm and we'll be fine (no speedometer recalibrations here!)

Okay, now that we have that understanding... a simple way to remember it is, for every inch you add on a rim, you want to subtract 12.7mm (one half inch) from the side wall (and vice versa for going with 1" smaller rims). That way you don't have to keep on calculating overall diameter...

In my case (and all of you as well who have factory speedo calibrations), stepping up to 18" rims from 215/55R17s...

We determined above that these tires have an overall diameter of 668.3mm...

We can safely use anywhere in the range of 648.25mm to 688.35mm

For a 16" rim:

195/65R16 = 659.9mm = 1.3% difference
205/65R16 = 672.9mm = .7% difference
215/60R16 = 664.4mm = .6% difference
215/65R16 = 685.9mm = 2.6% difference
225/60R16 = 676.4mm = 1.2% difference
225/55R16 = 653.9mm = 2.2% difference

For a 17" rim:

205/55R17 = 657.3mm = 1.6% difference
205/60R17 = 677.8mm = 1.4% difference
215/55R17 = 668.3mm = exact match
225/50R17 = 656.8mm = 1.7% difference
225/55R17 = 679.3mm = 1.6% difference
235/50R17 = 666.8mm = .2% difference
245/50R17 = 676.8mm = 1.3% difference
245/45R17 = 652.3mm = 2.4% difference

For an 18" rim:

225/45R18 = 659.7mm = 1.3% difference
225/50R18 = 682.2mm = 2.1% difference

235/40R18 = 645.2mm = 3.5% difference (not within our 3% range, but many dealers will say it's fine... and in the big scheme of things, it probably is) (watch clearance)

235/45R18 = 668.7mm = .06% difference (not common high performance tire size) (watch clearance)
245/40R18 = 653.2mm = 2.3% difference (watch clearance)
245/45R18 = 677.7mm = 1.4% difference (watch clearance)

255/40R18 = 661.2mm = < 1.1% difference (*WARNING* This fits our car, but does NOT clear the fenders... If you are dropped, or you often have people in your car or drive on bumpy roads, I DO NOT ADVISE GETTING THIS TIRE!)

For 19" rims:

225/40R19 = 662.6mm = .9% difference
225/45R19 = 685.1mm = 2.5% difference
235/40R19 = 670.6mm = .3% difference (watch clearance)
245/35R19 = 654.1mm = 2.1% difference (watch clearance)
245/40R19 = 678.6mm = 1.5% difference (watch clearance)

For 20" rims:

225/35R20 = 665.5mm = .4% difference
225/40R20 = 688.0mm = 2.9% difference
235/30R20 = 649mm = 2.9% difference (watch clearance)
235/35R20 = 672.5mm = .6% difference (watch clearance)
245/30R20 = 655mm = 2% difference (watch clearance)
245/35R20 = 679.5mm = 1.7% difference (watch clearance)

For all practical reasoning, I'm not going to go above 20" rims for our dear Solara...

Keep in mind, if the overall diameter is larger than factory, your speedometer will be slow (you will actually be exceeding the speed your speedometer shows). If the overall diameter is smaller than factory, your speedometer will be fast (you will be traveling slower than your speedometer reads).

Also keep in mind... Your speedometer is NOT 100% accurate (no matter how new your car is). There is always an undefined margin of error.

Anyways, I hope I've been of some help!

Last edited by theprodigy79 on Sat Apr 29, 2006 10:03 pm, edited 4 times in total.
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Postby x0swtextacy0x » Wed Oct 12, 2005 3:43 pm

ow my brain hurts that was a lot of math haa.... i think ima stick with my little 17". ... i wish i took my michelin pilot sport a/s tires off my max before i sold that thing. but then i'd have to get new wheels...hhaha too much for the first few weeks. ahh well.
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Postby PhreakdOut » Sat Nov 19, 2005 9:46 pm

This is one fantastic piece of information!! I used this guide for finding my latest set of tires and wheels.

Here is a tire calculator which is good for size comparisons. While tires will vary from manufacturer to manufacturer, this is a pretty good guide


For the Gen2 Owners, I can confirm first hand that the following tires do fit.

215/55/17 = OE Tire Size
235/50/17 = Slightly wider tire. Clears the strut housing and fenders on a lowered Gen2's with TRD springs.

Thanks for your posting!
225/40/18 = Diameter is too small. Increased gap between the tire and fender.
235/45/18 = Near Perfect diameter against OE size. Clears the strut housing and fenders on a lowered Gen2's with TRD springs.
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Tire Size Calculator

Postby PXLpainter » Sat Feb 25, 2006 10:00 am

Awesome info - thanks for providing it!

And for those of us who suck at math - here's a Tire Size Calculator:


...but that list you provided is great! :D

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Postby steven0930 » Sat Apr 29, 2006 6:59 pm

that was great info man
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Postby ASG14 » Thu May 25, 2006 10:36 am

On top of the other 2, here is another good link I always use for the calculations. Good writeup BTW. :)

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