Illustrations are courtesy of Wheel Vintiques. I would just link you to them instead of regurgitating this information, but since web sites come and go, so does their information and I wanted to make sure their crystal clear illustrations were preserved for my own use... as well as yours. In case you're wondering, yes, my wheels are Magnum 500's from Wheel Vintiques - I ordered mine through Summit. It's awfully tough to beat their prices.
What these drawings are for is to help answer that often asked question,
"what will fit?" First, you need to know how to measure the basic rim
width, then the backspacing... then comes the fun of measuring the suspension.
If you're looking to stuff a lot of tire in there, you also need to factor in
how wide the tire "pooches" out on the side walls and if you're
running normal suspension in the rear, you also have to worry about body lean.
Leave too little clearance and you can find your tire insides rubbing the fender
wells. Of course that's also a real good excuse to buy a rear sway bar.
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The most fundamental measurement is wheel
width and back spacing. This illustration makes it about as clear
as it can get. Notice a 15 x 7 wheel is 7" on the inside
of the rim. You have to remember to include that extra lip when
you're trying to figure out if the rim will clear everything on the
suspension.
The back space, though, is measured from the hub flange to the outside lip. When you're trying to decide if the tire will fit, too, the new
numbering system, for tires makes it a lot easier. I'm running
275/60/15 on mine which means that from sidewall-to-sidewall, the tire
is 275mm wide. "60" is the aspect ratio. Multiply
275 x 0.60 (60%) and you have how "tall" the sidewall is
from the rim to the tread. Divide that by 25.4 to convert to
inches, multiply it by two and add it to the wheel diameter to get total
tire diameter (28" for my tires). The actual radius of the
tire will actually be a little lower when it's loaded, but that's not
our problem just now. Now what if you're primarily wondering how much tread width you can get in the rear wells? I used 82% as a "typical" ratio for a tread to sidewall on a street radial. For my 275's, multiply that by 0.82, then divide it by 25.4 and I have just under 9" of tread. |
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Where all of that tire and backspace mumbo
jumbo comes into play is when you want to figure out with a plumb bob
and a tape measure whether your tires will fit. In this case it's
"measure twice - buy once" because every wheel manufacturer I
know of says that once you mount tires to their rims, their won't be any
returns. If you rush out and buy those 10" rims and have those
fancy M/T tires mounted on 'em, you're going to take a bath on them when
you find out they don't fit.
First, you want to know how far back the tire will come from the wheel flange. You need to know the wheel backspace (B), the total wheel width, and the tire width. For the wheel in the picture above, we have 8" total width, and if we stick with my tires, we're running 275's. To figure out how far back the total combination is, what I do is convert 275mm to inches (10.8"). Subtract the 8" wheel width from that (2.8"), and the divide the answer by two (1.4" - half the leftover pooches out to the front and half pooches out to the back). Now... this is what I call the tire overhang. I
don't know what the pros call it. You also have to check to make sure it fits within the lip of the fender. Make sure that the tire overhang is less than D. By the way... if you need me to walk you through it, you can figure out D from knowing the total wheel width, the back spacing, and measure from the axle flange to the fender lip. Subtract B from the total width to get C, then subtract that from the measurement you made to get D. Make sense? Final note on this measurement... don't just check your widths by
measuring level with the axle... this is where plumb bobs can be handy -
you need to imagine where the tire sidewalls will end up, project that
vision onto the inner and out fender wells and make sure that at the
smallest value of A and D that you'll be ok.
How's that for confusing? |
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I threw this in just for good measure... this just to illustrate how to get a measurement of what size lug pattern your wheels have and it gets you close enough in spite of not being a true diameter in the case of 5 lug patterns. |
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These next two pictures are something to
consider if you're doing brake upgrades (larger rotors for example) and
you're not sure if everything will fit inside the wheels you have or
want to have. Another thing they don't show is on front
suspension, you have to be careful that the suspension linkage and the
ball joints will fit inside of what you're trying to install.
One good example is a swap to install 11.75" diameter rotors on an older Mopar - this as far as I know forces a minimum wheel diameter of 15". Since I wanted to go to 15" wheels anyway, this wasn't a problem for me - but it's a great example of doing your homework when you contemplate changes.
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Figuring maximum front tire/wheel size is
also complicated by the fact that... yes... you can turn the front
wheels. 
So even if everything looks good from a clearance standpoint, you have
to worry about rubbing the frame or the fenders when you turn.
Let's just say that I got a tad carried away on mine. It's still
ok for the most part but you do have to be careful turning the damn
thing now  |
The only time the total tire diameter can be a problem is if you have a
lowered car or you're insane and running massive wheels 
Then, you have to pay attention to make sure the tires don't hit the top
of the fender well.
If you find yourself having trouble keeping track of what all you're
measuring, draw a picture. It's worth 1,000 words and could be
worth $1,000 saved, too.