BB 440 Mileage?

A big block with 4.10 gears will be a lot of fun, good luck keeping your foot out of it!

Seriously, if I had to WAG a guess I'd say 10-12. Do something to bump the compression and you might crowd 14 if you drive like a grandma and keep under 50 MPH. Bump the compression and re-gear and 14-15 should be no problem.