Well, that makes sense, and is something I hadn't taken into consideration.
The larger tires can result in a larger inch-measurment deviation, and still
give the same spec in terms of degrees.

    Toe in is measured from the pivot point, the axle, just like the
actual degrees in chamber, castor, and on the alignment machine the
toe-in would be .25 degrees for perfect:
http://www.billhughes.com/alignment.jpg and out to .45 to be still
within specs. If you triangulate that on a 33" tire it's over an eight
of an inch: http://www.billhughes.com/33tiretoe_in.jpg 
       God Bless America, ßill O|||||||O
mailto:LWHughes3rd@aol.com

Actually, I think the best bet is to make your measurements at the rims.
That way, unless your rims are bent you'll be setting the proper toe in for
the wheels and you have a nice hard, flat surface to measure from which is
always the same (7.5 inches from wheel centerpoint for 15" wheels).  I would
be suspect that either the tire tread pattern or sidewalls could vary too
much for the degree of accuracy you are trying to measure.

Here's a slick way that you can measure toe-in:
Take a spool of cheap fishing line and tie a nut to the end.  Wedge the nut
into the tread on the rear of one of the back tires about 1/2 way up from
the ground, then string the line around all four wheels being careful to
avoid any bodywork between the front and rear wheels on each side so that
the line goes across each wheeel at its midpoint and tape or tie it off
somnewhere so it is snug.

Make sure that the steering wheel is as straight as possible.  It can be
restrained in the centered position if you want.

Measure the distance that the line is held away from the rim due to the
sidewall bulging (depends on what you have for tires) and then cut 8
identical blocks sized about 1/4 to 1/2 inch wider than that distance.
Place double sided tape (thin carpet tape) on the backs of each of the
blocks and stick them temporarily on your rims under the fishing line.  On
stock Wranglers the front and rear track is identical.  On other vehicles
where the track is wider in the rear you can compensate by making the 4 rear
blocks thicker than the front ones by half the difference in the track spec.

Since the fronts edge of the wheels are supposed to be closer together, if
you gently pluck the line away from the front block until it still just
barely touches the rear block you can measure how much that wheel is toed
in.  Do the same on the other side.  Both sides should be the same amount
and the total of the two is the total toe-in.  Adjust the tie rod ends first
for the total toe, then adjust both sides the same amount (in opposite
directions) to get the same toe on each side, which will center the wheel
when driving straight ahead.

After adjustment, drive a little bit on a flat road and analyze the wheel
centering.  If it is not straight, make additional small adjustments (on
both sides, opposite directions) until the steering wheel is perfectly
centered.  When satisfied, run the line around again, measure and record the
values for future reference.

The rear wheel angles can be checked the same way, though there is not much
you can do about them short of centering the axle.

I've been able to do a better job setting toe-in this way (after replacing
tie-rods) than the local alignment shop generally does.

-Fred W

Back in 1964 when I went through Bear Mfg factory training in Rock Island,
Ill, it was more better to drive over the Bear "Scuff Gauge" . We used to
set the toe in @ 3 feet. This meant that the tire was dragged  3 feet
sideways for every mile (which was equal to 1/8" toe in - but was a dynamic
reading). The biggest benefit of using the scuff gauge was to drive over it
for the initial reading, then have several "hefty" people stand on the front
bumper & drive over it again (to check for toe change while braking) If the
2 readings were different then we would heat up the pitman arm & bend it up
or down until they were the same.