From 70-80 if you're floorign it in an automatic you should be in 2nd gear's top end. Our 2nd gears are good up until about 5.7k - 5.8k give or take. The top of our 2nd gear sucks, and unfortuantely you have to learn to trick an automatic into shifting into 3rd.
In runs at the track I've found doing manumatic with overdrive off to hit 2nd gear from 1st at 6k works as the best transition there, and then stick on your overdrive and throw it into "D"/drive and the car should be smart enough to hit your 3rd gear from 5.8k in your 2nd gear at around 75 mph. If you run any higher than that you're really not gainging anything and when you get into 3rd you'll feel third pick up better than your 2nd gears top end.
At least in my experiences.
If you're already at 70 and you gun it, you may just be holding in a higher gear and lsowly accelerating, it happens with our 1st to 2nd gear sometimes. Our 2nd gear basicly causes a lot of trouble, and you have to trick your car.
The best solution to this = get a manual
I've spent a lot of time leanring how to trick my transmission and getting the best feel for controlling my gears as I can, but sometimes the car just goes "ahhh nah, I don't feel like downshifitng today, sorry"
-chris