Buying Wheels Online Before Tires

[BuffaloXplorer]

Anyone out there know how to figure out what size tire (comparable to stock) i'd need when buying wheels first, then tires? I've found a few good "Wheels Only" deals online but i'm not sure what size tire i'd need on each rim size. I've seen the Miata.net site but that only compares your existing wheel/tire setup with another wheel/tire setup. I'd like to be able to go to tirerack.com and price the tires i'd need.

I.e A 20x8.5 wheel would require ______ tire size to compare to my stock 235/75R15's?

Is there a site or specific calculation that i can use? Thanks!

 

[Big Duke]

Well, I can't help you. However, I do basically have the same question. I already bought a set of 20's and I'm trying to find a good tire for them. The wheels are 20x8. Is it best to go to wally world or a place like dicount tire, tire kingdom, or an online store like tire rack? What have you all done in the past? (Especially those running a 20'' wheel Where can I getr what i'm looking for at the best price?

 

[93whitelightnin]

Ok,.......I'll try to explain this as best I can.

Example : 235/75R15

235 is the millimeters in width. This needs to be converted to inches in order to perform the calculations. Mulitiply the 235 x .03937,.....which equals 9.25195. Take this number and multiply it by the aspect ratio(height to width), which in your case is 75 This = 6.93896

Now,...... mutiply that number by 2, which gives you 13.8779
Add your wheel diameter (15), and this gives you an overall height of 28.88"

You can use this formula for any tire that is sized in millimeters.

For your 20" application, you could go with a 275/40R20 which specs out to a 28.66 high tire. This is about as close as your going to get (less than 1/4" difference)

Hope this helps

 

[the_tool_man]

Hi BuffaloXplorer:

First, you need to know the tire diameter of what you have:

Stock tire 235/75R15:
Tire section width = 235mm
Tire sidewall height = 75% of 235mm = 176.25mm
Tire diameter = 15in + 2 * 176.25mm = 28.88in (note the conversion of mm to in - 25.4mm = 1in)

Then figure what sidewall height you need with the new rim to have the same tire diameter:

New wheel diameter 20in:
Tire sidewall height = (28.88in - 20in)/2 = 112.75mm

Now try a few different aspect ratios to see what widths result. With an 8.5in wide wheel, you probably want a section width between 255 and 265mm:

40 series: 112.75/40% = 281.88mm
45 series: 112.75/45% = 250.55mm

Let's try a few and see what tire diameter results. I usually check Tire rack's search by size function to see if anyone makes a certain size:

255/45R20 -> 29.04in - very close, but not available
265/45R20 -> 29.39in - 1.7% off, 1 tire listed on Tire Rack
255/40R20 -> 28.03in - 2.9% off, not available
265/40R20 -> 28.35in - 1.5% off, not available

Not much there, so let's try 245 section width:

245/45R20 -> 28.68in - very close, but not available
245/50R20 -> 29.64in - 2.9% off, not available

Let's try 275 section width:

275/45R20 -> 29.74in - 3.2% off, 3 tires listed on Tire Rack
275/40R20 -> 28.66in - Very close, 7 tires listed on Tire Rack

275/40R20 would be very close to the same diameter, but a little wide for an 8.5in wide wheel. 275's are typically mounted on a 9in wide wheel. You don't seem to have much choice, though. If you're willing to tolerate more speedometer error, you could look at other sizes. I'll leave that to you.

Regards,
John.

 

[Big Duke]

Wow what an equation. I'm going to try and chew on this for a while. Definetly need a calculator. I was told that a 265-50-20 would be good. However, I'm wondering if that is as big as I can safely go. I don't mean to be repetitive but I want a big tire on this 20" wheel. I do have a tt and shackles. Any suggestions.

 

[BuffaloXplorer]

Now, if i was an edumakated man, i could understizand what the he-- you just said. No really, thanks for the help. I can work with this. HOWEVER, i just had the truck towed into the shop for a fuel system failure, and most of the $1500 i had set aside is now tied up.

 

[Hartman]

Simple answer:

The "true" stock equivelant is a 275/40/20, which is what I have. This size is rare and is only made by Pirelli and Continental. The Pirelli is the cheapest option at $230 per tire. Most people go with a 275/45/20, which is slightly larger than stock, and are $140 per tire.

 

[Big Duke]

Ok Thanks.

 

[the_tool_man]

Simple answer:

The "true" stock equivelant is a 275/40/20, which is what I have. This size is rare and is only made by Pirelli and Continental. The Pirelli is the cheapest option at $230 per tire. Most people go with a 275/45/20, which is slightly larger than stock, and are $140 per tire.

Tire rack has 7 different tires among 4 different brands in that size (275/40R20) ranging from Michelins at $311 to Pirellis at $158. See for yourself (watch for word wrap):

http://www.tirerack.com/tires/Compa...rue&pagelen=20&pagenum=1&pagemark=1&x=12&y=11

Tire rack has 3 different tires in the larger (275/45R20) size from $222 to $134:

http://www.tirerack.com/tires/Compa...rue&pagelen=20&pagenum=1&pagemark=1&x=13&y=10

Of course, Tire Rack isn't the only place to get tires, so there are probably others out there. I'd go with the correct size if it were me.

Regards,
John.

 

[Hartman]

Hmm I guess they added some since I bought mine. They are still quite expensive though, except for the Pirelli snow tire.

 

[Big Duke]

Of course, Tire Rack isn't the only place to get tires, so there are probably others out there. I'd go with the correct size if it were me.

What do you mean the "correct" size? Like I said I want the biggest tire I can get on the X w/o rubbing and sticking out.