Isn't there a way to jack up one rear tire off the ground (tranny in nuetral), turn it one revolution and count the number of times the drive shaft turns? If it turns 3 times then you could guess you got 2.90's instead of the 3.90's. I am curious myself but this kind of makes sense to me. I could be wrong but isn't that what a ring gear does? 3.90's would require the 3.9 revolutions input to the diff in order to get one revolution out?
Bo
Bo