Your gauge isn't any different than the wheels, there are allowable tolerances. Try to imagine the factory where the wheels or wheel gauges are made and a the quality of the laborers. Now, imagine an employee saying boss its 1/2 a MM to big, lets throw it away, how would a business survive like that. There are a ton of variables as to why there are different sizes that are supposed to be the same size. You need a CALIBRATED micrometer , not your eyes to see the difference. I have seen metal, using a micrometer on aircraft parts, expand and contract that much with small temperature changes. Have you ever heated/chilled a part for removal or installation?
Your nifty plastic gauge is not the end all of this topic, sorry.
Ask any engineer that deals with manufacturing of metal parts.
If you want me to go on about tolerances I can but, the point is I use the wheels.
