If highways didn’t post miles by state…

[Flint1979]

Using I-75 at exit 150 it would come out to be exit 1541. I'm only 246 miles from the northern terminus of I-75. My mileage for I-75 comes out to be 1,787 miles.

[Flint1979]

Depending on whether I-80 or I-94 took priority, it would be either exit 2133 or exit 1265.

I think in that area I-80 has priority because at least on the Illinois side it uses I-80's exit numbers but then they reset at the state line for both highways since they are running concurrent. But at one end you have to exit to stay on I-94 and at the other end you have to exit to stay on I-80 so who knows.

[FrCorySticha]

My answer would depend if you count CA 15 as part of I-15 or not.

I-15 alone: Exit 1295
CA 15 and I-15: Exit 1301