Measuring and Adjusting Toe

Isn't "Toe" based on tire diameter? If yes then measuring toe at the wheel will be inaccurate.

Toe is an angle against what is directly forward from your car. Measuring 1/10th or 1/100th of a degree is exceedingly difficult if you were to try with something like a protractor. The further away from the steering axis we go the easier it becomes to measure. The wheel is an easy thing to measure against, so its common to use it.

In general people avoid trig, so they simply state toe as a distance measured at the wheel, but you can convert to angle if you prefer, or make a large change in wheel diameter.

Yup, you can work out the angle with basic trig' from the toe distance and wheel diameter, and vice versa.

You can make up a simple conversion chart for your notebokk, or electronic device, for the difference between the rim distances as a quick reference, especially if you also add values for other wheel sizes that may be used, such as 15" dirt and 17" tarmac for the rally chaps.

Thanks for the inputs. So I did the math and here is why I question where the specification comes from (toe of wheel or toe of the tire). My tires are 245/40/17 which gives me a tire diameter of 24.7". If I setup my car so that I have 1/16" (.0625") toe (each wheel 17" dia) this gives me a total toe of 1/8" (.125"). Based on the math my 24.7" tires will have a toe of .091" or a total of .182".

Yes. Measured at the tread, horizontally, 180 degrees apart