You experience is very different from mine. I wonder if something is wrong with your car.
You say you get 4.9 mi/kwh -- with a 24 kwh battery that means you should get 118 miles on a full charge. If you're only getting 70 then your battery is only holding 14 kwh, 60% of its rated capacity.
Personally, I routinely get 120 miles out of a charge when I stay off the freeway and don't need climate control. Just yesterday, for example, I made an 80-mile round trip to the airport using just over half of the battery, driving on surface streets, averaging about 45 mph. The car says I averaged 5.4 mi/kwh which should get me nearly 130 miles.
I do agree that at 90 mph you're not going to get 70 miles. I didn't mean to imply that, though I can see that's a reasonable conclusion from what I wrote. One of these days I should test my range at 80 mph (freeway speed around here). Rarely do I drive more than a few miles on freeways, though, so it's not all that relevant to me.
Anyway, I think you've suffered some really serious battery degradation, and you should get it looked at.