Gas Milleage and Gas Grade

[El Diablo]

Two newbie questions:

What type miles per gallon are ya'll getting on your GZs when city driving only? Salesman said should get around 70? I'm still on my first tank. Since there is no gas gauge, guy at shop told me to set tripometor and drive until I run out of gas. note the miles, go to gas station on reserve and reset trip to around when to get gas next time. I've gone 160 miles so far. By my calcs, should have another 22 miles or so. (3.4 capacity less 0.8 reserve = 2.6 X 70 = 182 miles).

Salesman said only run premium gas in it. I read somewhere, maybe this forum, that was hogwash. Manual just says minimum of 87 octane. What's the consensus among GZ riders?

Thanks all.