Should toe-in be checked at the wheel or outer diameter of the tire. This is with 38" Continental MPT 81. Searching the web shows most checking at the tire but with stock tire sizes. Trying to pry info from local alignment shops all I could get from them is that they check toe in at the wheel. I'm using 20" custom wheels instead of the stock 19.5" wheels also. I did ask elsewhere also but received no replies. TIA

 

Wheel alignment is measured at the wheel rims. I had an old 75 Dodge power wagon many years ago that had bad tie rods, and after replacing the tie rod ends I used two fiberglass 5/16" rods to measure the wheels across the rear of the front wheel and then the forward end of the front wheel. I marked where the fiberglass rods overlapped to give a reference point and then adjusted so that the distance between the wheels was the same front and rear, and toe-in was set at 0. The truck ran truer and was probably easier to keep running down the road straight without much correction than any truck I have ever owned.

 

We mount our plates on the wheel when we align our car/trucks. I would use the wheel. I would also be very careful about putting too much toe in on a straight axle truck (don't know what vehicle youre working on).