<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Canarsie:
Interstate 84 in the western United States would become Interstate 86 between Portland, Oregon and Pocatello, Idaho (as there is already another Interstate 84 in the eastern United States); and Interstate 84 between Interstate 86 and Interstate 15 would become Interstate 17.</font>
There's also both a Western and Eastern I-86; the Eastern I-86 will eventually follow the NY17 corridor. I think that currently the only stretch signed is between I-87 and Binghamton. I'd call the Western parts I-88 and call the finished NY17 corridor I-86 while renaming the present I-88 in New York to I-186.
You also have Western I-76 (Denver to I-80 just over the Nebraska line) and Eastern I-76 (PA Turnpike and Ohio extension). This one is easy; rename the Western I-76 to I-170 or I-180)