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 dont 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 its 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
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.