The Trailer Wheel Center Cap # 319EZ may be a little too big if you measured right at 3-1/8" for your hub bore. Since this is a fairly precise measurement I recommend taking it again and possibly use a digital caliper like part # PTW80157 if you have one handy to make sure it fits.
I do not advise grinding the hub bore out at all as that could compromise the integrity of your hub. view full answer...