I'll just leave this here.
Mythbusters: drafting 10 feet behind a big rig will improve mileage 39 percent — Autoblog Green
The chart at the top is useful (Do what you want with the commentary below it). Granted its at 55mph not 70 so the percentage gains would likely only increase for your purposes. Quite likely drafting at 10-20 ft would boost your mileage beyond any slower non drafting speed.
However from another site I read that at 55mph at 10 ft you have .143 seconds to react. Not sure how they calculate that. I mean tractor trailers cant stop as fast as a car. Either way I hope your paying very close attention when you do this and bring a clean pair of underwear just in case.