See this link to Convert Toe Degrees to Inches
Toe measurement in degrees is simply the angle between the vehicle centerline and the tire. Toe measurements in degrees are more accurate than measurements in inches because inch based measurements are affected by tire size while the angle is independent of tire size.
The Honda AP1 S2000 Shop Manual calls for 0.25 inch of total toe-in at the rear wheels. Toe-in in inches is the difference between measurements A and B in the diagram below. Shop Manual toe-in in inches is measured on the tire tread at the axle level.
0.25 Inch (0.58º) of Total Toe-In
Measurement A - B = 0.25" or 1/4" total toe-in. Note 0.0625" = 1/16" and 4/16 = 1/4
The Shop Manual 0.25 inch of total toe-in is made up of:
0.0625" measured at leading tread of the left tire + 0.0625" measured at trailing tread of the left tire +
0.0625" measured at leading tread of the right tire + 0.0625" measured at trailing tread of the right tire. 0.0625" x 4 = 0.25".
When measuring toe in degrees you do not need to double the measurement for the leading and trailing edge of the tires as you do when using inches. If one rear wheel measures 0.29º of toe-in and the other measures 0.29º, then total rear toe-in is 0.58º, not 1.16º.
AP1: 0.25 inch (1/4" or 6mm) of total rear toe-in is equal to 0.58º (decimal degrees) of total toe-in.
AP2: 0.14 inch (3.7mm) of total rear toe-in is equal to 0.33º (decimal degrees) of total toe-in.
If you use string to measure toe and use the wheel rim for measurements your numbers will not match up with the Shop Manual because Shop Manual toe is measured at the tire tread. You can get pretty close by decreasing the tire tread measurement by 25% to approximate the correct rim measurement.
If someone says they run 1/4 inch of toe-in you need to ask:
Is that total toe-in? (total for both wheels on the axle, or just a single wheel?)
Where was the 1/4" measured, at the rim or tire tread? (tire tread is the norm)
Was the 1/4" measured on one side of the tire, or both sides (leading & trailing tread on both tires = normal total toe)
By Rob Robinette