Originally Posted by Callipygous
I am not a car person, so forgive me if I'm missing some information, but as a math person, this particular bit jumped out at me:
"First, true spec is measured midway up the tires. If for any reason you cannot measure half way up because of bumpers or skid plates or whatever, take the front and rear measurements 1/4 of the way up the tires, then double that to get the true toe as it would be in the center of the tires."
This seems to be off due to the geometry of a circle. By the time you reach "1/4 of the way up the tires" you would have covered (sqrt3)/2 of the horizontal distance, which is about 86%. If you then multiply by 2 you are finding 172% of the number you are looking for.
If instead you measure 1/4 of the diameter toward front/back from the center of the tire, that would yield a number which you can simply multiply by 2.
You're right, it's 1/4 of the way along the radius
. However, even that's not going to be exact. Tires are not a perfect circle when they are loaded by the weight of the truck. My tires flatten over 1" on the loaded side of the tire. I have always taken toe measurements based on the center of the hub to the ground and made my marks on the tires at whatever I measured (typically around 15-16" depending on how much tread I have left and tire inflation psi).
More importantly, the goal is to have total toe at 0 (if your driving habits are like mine). At most, alignment shops will do 1/16" toe in. If you set your total toe at 0, it's not going to matter where you take your measurement from. If your goal is 1/16" total toe in, and you measured 1/32" (and then "double" it as I mentioned) at 1/4 of the way up the vertical distance, you're only going to be off by about 1/32"..and still minimally toe'd in. At such fractions of an inch, it's not really going to matter. There's a larger variance by just not putting the tape measure in the same spot.
Along the same note, I originally drafted this such that anyone who had basic tools in their garage could do it. Since then I've stopped measuring toe with this manual method. I bought a set of Toe plates about a year ago and have been using those since. Way faster and definitely more accurate.