[Warren]

Ill admit it, I am an Idiot. Can someone (or a collection of someone's) explain tire sizes to me?

I have purchased a second set of back (stock) rims, and plan on putting drag radials on them. Finding some tires to fit these rims has proved to be less than easy. I was told that a 245/55/17 fits perfectly. What the hell does 245 mean, can I go 275? What does 55 mean, can I go 45? I think that I cannot go 18, without rub. Not sure. I am a tire idiot. :drink:

Oh, and if our Aussie firends read this, I meant tYre.

 

[ZillaGTO]

The numbers are width/aspect ratio / diameter (which is also the size of the rim)

So a 17 means only tires with a 17. Tire width depends on rim width so 8 inch rims can theoretically hold less than 9 inch wide rims. The side aspect ratio will change the ride comfort and handling by essetially shortening or widening the sidewall.

For drag radials you likely want something different than autocross, so I will let someone else who does those things continue this.

 

[Warren]

All I want are tires that I can switch out with the stocks when I go to the track (or need extra stickys for challenges)

 

[goatee]

Zilla simplified it best, but what it means is how big the overall dimension of your tire is... makes a difference for speedo and odometer. Sounds like these are track specific tires, so you can play with the aspect ratio/wheel/rim size to affect the drive ratio to squeeze a bit more tailored performance from the car.... smaller overall diameter would be better for accel, larger for top speed.

last number is obviously the diameter of the rim.
first number is the width of the tire in MM.
the second number is the height of the sidewall compared to the width (aspect ratio)

therefore, a 275 40 has a sidewall height of 275x.40= 110mm

Multiply that by 2 and add to your diameter to get the oveall diameter.... as far as the width, you need to make sure the rim is wide enough for the tire, and that seems up for debate.

 

[mistermike]

For drags, I'd stick with the 17's and get the biggest kahuna that will fit without rubbing. The extra sidewall will help your launch. The higher profile may let you "cheat" a little on the width, as long as you stay in a straight line and don't overinflate. Florit has drag radials, but I don't recall what size he got.

 

[Warren]

For drags, I'd stick with the 17's and get the biggest kahuna that will fit without rubbing. The extra sidewall will help your launch. The higher profile may let you "cheat" a little on the width, as long as you stay in a straight line and don't overinflate. Florit has drag radials, but I don't recall what size he got.

Yeah, Florit has the 245/55/17's, he is who I bought the stock wheels from. The problem I am coming across is that I cannot find anyone who stocks, or can even GET drag radials in that size around here. Internet searches on tirerack have not done much for me, since I am still an idiot when it comes to what I need. I just want to go very fast in a straight line down the dragstrip for 1/4 mile. I want the best tire for doing that, with a stock rim.

 

[kjac081]

therefore, a 275 40 has a sidewall height of 275x.40= 110mm

Multiply that by 2 and add to your diameter to get the oveall diameter.... as far as the width, you need to make sure the rim is wide enough for the tire, and that seems up for debate.

To make it easier, divide the width by 25.4. that will give you width in inches.
then mulitply by aspect ratio, multuply that number by 2 then add to your rims size. you now know OD.
245-45-17 = 245mm /25.4 is 9.65". 9.65 x 45% is 4.34. (this is sidwall height)
4.34 x 2 is 8.68 added to 17 is 25.68.
to find a replacement tire, the general rule of thumb if keeping the same rim diameter is plus 10 and minus 5. add 10 to the width and minus 5 from the aspect ratio. this works in most cases and ends up pretty close.
255-40-17= 255mm /25.4 is 10.04. 10.04 x 40% is 4.02
4.02 x 2 is 8.04 +17 = 25.04.
In some cases there may not be a tire made in the size need to follow this rule, so it takes some work to find whats right. It sounds complicated but really is simple for a tire expert.
fwiw, i sold tires for about 4 years at a national chain.

 

[kjac081]

also, you can check the manufacturers spec sheet on the tire to get the exact width and OD. The mathematical formula works, but real world manufacturing yeilds small differences.
also on the spec sheet for a tire it will list the approved rim widths for each size.
for a stock 8 or 8.5 rim 255 is probably the biggest i would go. 275's will fit though.
heere is the specs for the nitto 555, and as you can see the sizes for 17's go from stock 245/45 to 275/40. the OD is close to the same but the 275 is a full 1.3" wider and is recommended to be on at least a 9" wide rim, with 9.5 being ideal.
http://www.nittotire.com/popup_555r_sizes.asp
you may try BFG drags, they may offer different sizes.
here is the bfg link
http://www.bfgoodrichtires.com/bfgapp/catalog/tires/gforcetadragradial.jsp
they have two tread patterns available so click the link that says 'Available in alternate tread pattern'

 

[phobos512]

Here's a nifty little gadget for you guys to try out - you put in the original and new tire specs and it will compare the sizing for you, show you the difference, etc. Just ignore the Miata part.

http://www.miata.net/garage/tirecalc.html