Rim and tire lessons, please!

[Chris6060]

Hey guys, I have a few questions about rims and tires.

As far as tires go, say you have a set of 265/70/R17 tires... The 265 is height (but where is it measured from? and in what unit, mms?), the 70 is width and the R17 is rim size, correct?

I've just noticed on sports cars that the first number seems to be width, not height... So I'm all confused.

As far as rims go, our trucks are the 6x5.5 bolt pattern, correct? And how do offsets work? Is a negative offset further out from the body, and a positive is further in, or what? And where is offset measured from?

Thanks guys! Sometimes it's the simplest things that a guy doesn't understand.

 

[bluex]

On any metric sized tire the first number is the section width in millimeters. The second number is the aspect ratio, an it is a percentage of the section width. The last number is the rim size.

So in your example of 265/70/17 this tire would be 265mm or 10.433" wide. The aspect ratio is 70% of the width, or 185.5mm/7.3". This is the sidewall height of the tire. To get the overall height of the tire you take the rim size of 17" in this example, double the aspect ratio an add them together giving you an overall height of 31.6"/802.64mm. ( 17+7.3+7.3 )

P at the beginning means its a passenger car tire an LT stands for light truck. The R before the rim sizes means radial construction.

 

[bluex]

4x4 GMT400 trucks are 6x5.5" bolt pattern yes. The 2wd ones are 5x5". In 95 the lug nut threads changed over to metric (M14x1.5) from standard (1/2"-20).

On rims offset an backspacing give you the same information just in different ways. I'll start with offset since that's what you asked about. I'll use an 8" wide rim as an example. Offset is the distance the mounting surface of the rim is from the center of the wheel. So on an 8" wide rim with an offset of zero the mounting flange is exactly in the center of the wheel. Offset is also done in millimeters, so our 8" wide wheel is 203.2mm wide.

A positive offset brings the center of the wheel closer to the outside edge of the wheel. So an offset of +16 would be figured like this. The center of the wheel is at 101.6mm, a positive offset of +16 would add 16mm to that moving the mounting surface of the wheel towards the front of the wheel 16mm to a measurement of 117.6mm

A negative offset does just the opposite an moves the mounting surface towards the back of the wheel. So a -16 offset would take the same 8" wide rim an move the mounting flange from 101.6mm to 85.6mm.

So depending on the stock rims offset a negative offset will move the rim/tire out away from the vehicle an a positive one will move it in. Confused yet?

 

[bluex]

Now for backspacing. This gives you the same information as offset just a different way. It is usually given in inches. It is the measurement from the rear of the rim to the mounting surface.

Let's take our 8" wide rim again. If it has a 4" backspace then it also has a zero offset. Taking our earlier examples the rim with the +16 offset would have a backspacing of 4.63" (117.6mm=4.63" )

The rim with a -16 offset would have a backspacing of 3.37" (85.6mm=3.37" )

This shows how to measure backspacing.

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Hope that clears it up a little for you...

 

[Chris6060]

That was an awesome explanation, bluex!

I have BFGoodrich All-terrains with the 3-ply sidewall on my truck at the moment in an LT265/70R17 and I really like them, other than the fact that they throw a lot of rocks. But my rear ones are getting pretty worn (first owner towed a fifth wheel with a half-ton everywhere... And I like doing the odd brakestand, I'm still a teenager ), so I don't know whether I should just replace the rear ones, or rotate them and save for all 4 and step up to an 18" rim or what. If I got some 18" tires, I could save up for BMF Novakanes, but that would totally change the style of the truck as it is, and I'm not sure I could pull it off (but god DAMN IT I love those rims). I'm considering some Mickey Thompson Classic III's also, 'cause they're for one, almost 1/3rd of the price of the Novakanes and I think the chrome ones would fit this style of truck a little better.

I might stick a set of my buddies rims with 33's on my truck that look like black Classic III's and see if I like them. I think I'd end up with chrome though.

I also have a set of cheapie Core Racing rims that came with the truck at the moment that have the perfect offset for a stock height truck in my opinion (and I actually really like the look of the rim on this particular truck), but they're getting corroded pretty badly. It was already pretty bad when I bought the truck.

When I replace them with hopefully some Mickey Thompson Classic III's, I wanna maintain the same offset. So I'm trying to figure out what the offset is on my rims.

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This is the truck with the Classic III style rims, but a cheapie version that are total crap. It's an '09 and the rims are pretty much all gross and corroded already.

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[tylers88]

Blue good job, only thing I have noticed is most wheels are 1" overall wider than advertised width 8" is bead to bead so most wheels will be 9" total width making zero offset and 4.5" BS dead on for instance the 18x9s I was looking at for my 04 are 0 offset but 5" BS, otherwise info was spot on

 

[bluex]

All rim widths are measured from bead seat to bead seat, not on the outside.

 

[tylers88]

All rim widths are measured from bead seat to bead seat, not on the outside.

Yes, where the extra overall 1" extra width so that effects BS by 1/2" I didnt specify clearly rereading my other post, just giving some heads up to others that see this like if they buy a set of used wheels and just throw a tape measure on them