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Will this Fit? Tire Sizing and Wheel Spacing Explained

The math behind tire and wheel sizing

Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby Onlyina_Chevy [OP] » Oct 29 2011, 2:14pm

There has been a lot of redundancy in the wheels and tires section with many questions about tire/wheel sizes, especially in regards to, “What is the largest…” The truth is, there is more to picking out your next set of over-sized tires than height. Technically, you COULD fit a 35” tire on any of the 2007-11 2WD or 4WD Sierra/Silverados, the problem is that at some point you have to turn. Anyone that has every replaced a door in a house knows that you don’t measure the space between each side of the frame, and then stick a door in there that is that exact measurement; it has to have room to clear the frame due to the thickness of the door. Please view the drawing below of a door in a frame:
Image

The center of rotation is at the hinge depicted from my not so professional looking black circle at the top right. As the door would be opened (following the arrow) the door wood be wedged in the frame. Keep this in mind as we look at the upcoming visual aides.

The OEM tires on my 2009 Silverado are sized at 265/65 R18. The first number, 265, is the measurement of the cross section (section width) of the tread; 265 mm or 10.43 inches. The second number, 65, represents the sidewall height; it is 65% of the section width. In this case, 265 x 65% (or 0.65) = 172.25 mm or 6.78 inches (measured from the bead to the top of the tread). And lastly, R18 is the rim size. If we take 18 + 6.78 + 6.78 (the latter two are repeated because the tire runs all the way around the rim hence giving you top and bottom sidewall) which returns an overall height of 31.56 inches, which is the height of the OEM tire.
Image

Now listen closely because this is very important, not all tires of the same listed size are the same size. That is to say that a Nitto 265/65 R18 may be a slight bit taller or shorter and wider than or not as wide as a 265/65 R18 Goodyear. We live in an imperfect world, and that’s all I’m saying about that.
After installing a Rough Country 2” leveling kit, I was ready to put a slightly larger, wider tire on my truck. The gists of everyone’s questions are related to whether or not the tire will rub the inside of the wheel wells or the outer edge of the metal of the fender and/or bumper and front air dam. This is where the door picture above comes into play, as well as that odd red diagonal measurement you see on the picture of the OEM tire. The tire may be 31 inches tall, but it is 33 inches diagonally. It is not a random guess; there is actually a mathematical equation to figure it all out.
Think back to high school (for some this may be current or recently; for others, like me…ahem, it has been QUITE some time) and those famous words your Geometry teacher told you:

“Math is something you will use for the rest of your life!”

In order to figure it all out, you have to think of your tire (from the top view) as a rectangle. Once you have established that, you should imagine it as two right triangles with a common hypotenuse (the side opposite a right angle). We use the ever-so-popular Pythagorean Theorem (a^2 + b^2 = c^2) then find the square root of ‘c’ for the calculation. Using the OEM tire, we use the overall tire height and width to solve for the diagonal length as follows:

(800^(2)) + (265^(2)) = 710,225 mm
√(710,225) = 842.7 mm
Convert this to inches (25.4 mm per inch):
842.7 / 25.4 = 33.1 in

The aftermarket tires that I put on my Silverado were 295/65 R18. The tire height from the rim to the top of the tread was 7.5 inches (twice) plus the rim size came to a total of (18 + 7.5 + 7.5) 33 inches with a section width of 11.6 inches. This yielded me diagonal measurement of 35 in.
Image

The problem that I got into at this point is that the overall length at the bottom of the wheel well is a mere 36 inches.
Image

One would think that one inch is good enough, but like I mentioned before; YOU MUST TURN!
Image

And it is not only turning on a flat street, sometimes you may turn into an inclined drive, pop up a curb while turning, or do the unthinkable…go ‘4 wheeling’. These are the times that you must make sure you have more than enough room in the wheel well because as the suspension sucks up into the body of the truck, the wheel well gets narrower, but the tire does not.
Image

Last thing to discuss is wheel width and backspacing. Obviously if you have a 10” wide tire on an 8” wide wheel, the tire will not reach its full width because the rim will restrict it. Well the section width of the tire will be what it is supposed to be, but you will have more of a ballooning effect on the sidewall. However, if you have a 10” wide wheel, the tire will reach its full width including a reduction in the ballooning effect. But with a 10” wide wheel, you may run into other problems of rubbing the wheel on the different parts of the suspension, plus depending on the backspacing of the hub, you could run into the problem of the tire rubbing the fender, bumper, and/or air dam.

Back spacing is simple to understand. If your hub is dead center between the outside and inside of the wheel, then you have zero (“0.0”) offset. If the hub is pushed out toward the side of the truck you have positive offset (+ x”), and this would push your tire further under the truck. On the other hand, if the hub is pushed in toward the middle of the truck, it is called negative offset (- x”). This negative offset will push your tires to the outside of the fenders. Here’s a diagram showing all three:

Image

This is something to remember when lifting a vehicle and installing larger tires. As you raise the truck, you lose your low center of gravity causing a possible rollover when making a quick turn or if you get your truck in a precarious position while ‘wheeling’. A good rule of thumb is, if you go up; go out. This will make the truck more stable during extreme moments. A negative offset, if it doesn’t cause problems with the suspension, will push the tires out to regain the stability you may lose as your raise your truck.

Useful links for backspacing (offset), lug-patterns, tire sizing, and other wheel related things:
Backspacing Charts:

http://www.streetdreams.org/images/conversion.jpg
http://i47.photobucket.com/albums/f159/4DRLS2/TBSS%20Wheels/rimbackspacingchart.jpg

Lug Patterns:

http://www.lugpattern.net/Chevrolet_GMC_Bolt_Patterns3.htm

Tire sizing (calculator):

http://www.1010tires.com/tiresizecalculator.asp

Tire and wheel fitment guide:

http://www.wheelsforless.com/GM_Truck.htm
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby codysown » Oct 29 2011, 3:32pm

Nice Job. This is the best write up I have seen about this and have covered more than anything I have read in the same post. I think this should have a Sticky on it. It is definitely and must read for those who are trying to figure this out. Thank you.
Last edited by EKinMN on Oct 31 2011, 3:36am, edited 1 time in total.
Reason: MODERATOR NOTE: It is stickied. Thanks.
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby jepics » Nov 02 2011, 6:49pm

Thanks to Onlyina_Chevy for this excellent write up & to EKinMN for making it a stickie - great material to have as a handy reference. Two thumbs up!
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby ricka182 » Nov 02 2011, 7:05pm

Awesome post! I actually cringed when I saw your mention of Pythag theory thing.. I hated that in high school, but it makes sense.

So if I have stock rims at 7.5", and wanted to make the truck stance wider, I would want a negative offset. Is there anyway to do this without replacing the rims themselves? or did you answer that above, and I'm lost?....
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby Onlyina_Chevy [OP] » Nov 03 2011, 11:26am

To tell you the truth Ricka, the offset thing confuses me too; I think it should be the opposite of what it is.

To answer your question, you COULD use wheel spacers which fit in-between the wheel hub and the drum/rotor which would push your wheel outward a tad bit. I have never used them, nor would I use them because they are now an extra block, per sae, added to what may give me the most security about my vehicle (wheels are tightened to the lug nuts as their specific torque rating). I don't know exactly how to explain it, a feeling, but I would feel less secure having an extra removable part on my wheels. 6 lugs are enough without worrying about 12! Here is an example of wheel spacer on a 5 lug that is now a 10 lug:

Image

This would definitely give you your wider stance, but at some point if you decided to put a wider tire on your stock wheel (plus have your spacer), then you have to figure out that if it would have originally rubbed on the inner plastic within the wheel well, that now it will rub the fender, bumper, and/or air dam. I'm tellin' ya, the plastic is far more forgiving than the sharp metal (bumper or fender edge) with regards to rubber.
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby SSGBulldawg » Nov 03 2011, 12:15pm

Excellent write up my friend!! if i would have had this about a month ago it would have saved me alot of headaches lol. im in the process of buying the parts for my 17"-16" switchover on rims, body lift, spacers and 35x12.5's and i believe my calculations are correct and will fit. and using your method...... it will just BARELY which is what i want. once again.... Excellent write up!
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby Vortec6200 » Nov 03 2011, 12:36pm

Fantastic, excellent write-up Onlyina!! This will make my life much easier when I go get new larger tires with the RC leveling kit installed. Thanks so much. If we were in school, you'd get an A+ from me.
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby JennaBear » Nov 07 2011, 11:03pm

Great write up!
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby smokie3883 » Nov 20 2011, 7:12am

very nice on the explaining for people. bigger tires is always nicer, but them danged ol turning and bumps can really make ya go "crap". quick note for anyone that reads this. you can for a 100% fact put 15 inch rims on the front of a 2000 silverado. yes it rubs on the turns and if you hit a pebble, it will scrab the wheel well, but they do fit. the back cant hold them, have to have the 16+ or get a smaller break job done (no clue why you would downgrade breaks) but whatever. but i do not suggest putting them even on the front. i know it is eating my rotors horribly. i can see the marks on the rims and rotors and hear them scrapping, but they get the job down for a short time. so, if you have to, 15 inch rims will work, just not a smart idea.
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Re: Will this Fit? Tire Sizing and Wheel Spacing Explained

Postby robeward » Dec 10 2011, 2:58pm

Awesome. Makes perfect sense. I would have done the measurements and thought if there was space then everything would be fine. Not always the case.

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