How To Measure For Wheels
Measuring for proper wheel sizes is a popular question. With wheels costing a bundle, and the fact that once you mount tires, you own them, it's a good idea to get it right the first time. So far, all the letters I've recieved have questioned the size of rears, so I'll cover this here.
To find the proper wheel and tire sizes for your car, you must first determine how much total room you have to work with. The easiest way to do this is to hang a plumb-bob from various points to the floor, mark the floor, and then you can measure these marks without any parts interfering. Before you measure, make sure the car's weight is on the axle, so it's in its ride-height position.
In the drawing, drop a line from the inner wheel lip to the floor and put a mark. Next drop a line from the frame-side, at the closest point in the well and put a mark. Dont forget about your fuel or brake lines if they are in the well. Last, drop a line from the wheel mounting flange and put a mark. These three points are all you need to find the wheel/tire sizes.
First, measure across the outer two points (C), to find the total clearance you have to work with. From this distance, you should subtract a fair amount of clearance on each side so the tire doesn't rub as the car leans. Around an inch of actual clearance on each side is about the bare minimum, due to sidewall flex in tires. The more air in a drag-type tire like the MT, the wider the sidewall gets, so don't skimp on this clearance measurement, or you'll be hammering your wheel lips to get them to work. Add some extra if you can get away with a smaller tire. 1.5" would be real nice. I understand that you want the biggest tire possible, but even I ended up hammering the wheel lips on my own car after it was painted because I "thought" I could get away with less. Choose a tire that has a side width measurement matching your new measurement. Assume C = 14.5. Take away 1.5" on each side for clearance and you have 11.5. You can assume that this tire will really be around 12.5 total width due to sidewalls bulging. That gives you an honest 1" on each side depending on tire pressure, which will most likely be enough for normal street driving.
Now, you want to center this tire in the wheelwell, to allow the most clearance on each side as possible. Assume here you're using a 10" wide rim. You want to center 10" in the wheelwell when it's mounted. If measurement (C) was 14.5", then you need to divide the extra 4.5" between both sides to get the rim in the center. So, you can have 2.25" of clearance on each side of the rim to have it centered in the well. Now take the measurement (B) and subtract that 2.25" from it. Assuming here that (B) is 8", taking 2.25" away will give you 5.75" of backspacing. So, in this example, you can fit a 10" wide rim with 5.75" of backspacing, and it will be perfectly centered in the well. We all know that certain backspacing isn't available, so assume that you have to go with the closest offering, which might be 6". This will move the rim .25" closer to the inside of the car, leaving you with only 2" from the rim edge to the inner well. With our 11.5" tire, we figured it'd be 12.5" total, so centering 12.5" on the rim would give us an additional 1.25" on each side. Adding this to the rim, you'll find that you now only have .75" clearance on the inside, while the outside is now 1.25". You can see that when using the biggest possible tire, .25" is alot. Like-wise, if you go the other way, getting 5.5" of backspace, you'll only have .75" on the outside. When you get under 1" of actual airspace between the tire and the well, you're getting very close, and risk cutting the tire when the car leans.
As an example of this, I have SS springs on my 69 Dart, which are very stiff. The car doesn't lean at all around turns, but in some intersections when the intersecting road is at a weird angle, the rear has no choice but to lean one way, and my MT Pro 12.5's rub. I had 14.5" of clearance and assumed 1" on each side from the tire size was enough. I had to hammer the wheel lips, and slightly hammer in the tack welds on the inner wheelwells. They now just barely touch during the worst conditions, but it was much closer than I had thought, and if I had a car that I couldn't "hammer" on, those tires would've been wasted money.
Obviously, radials will have different sidewall characteristics than bias ply's, but whatever tire you want, call the manufacturer or supplier and ask about total widths.
How many of you would return or sell the tires if they didnt fit? I bet every one of us would try to hammer or trim something to get them to work, and if they didnt work, you'd now have a dented car and still no tires. It's worth waiting on hold to get the tire info.
A last point to keep in mind is tire diameter. This is merely a matter of measuring from the center of the axle to the closest point in the well. It's good to first decide on maximum width so you know how far out the tire will be. Sometimes an inch of height is more important than width, so you may have to use a narrower tire to get inside an obstruction for the height. Another note here is that usually, the front lower edge of the wheel opening is the limiting factor. Its good to keep this in mind when moving your springs...Moving the rear back a little can give you more clearance for a taller tire.