MacCool

It's not a commuter. It's not a luxury car with roomy, plush interior and Carnegie Hall sound system. It's a rear-wheel drive sports car, it handles poorly in the snow, it's small, has no luggage capacity, is noisy, and if you drive it to conserve gas, all you have is a small, cramped, noisy, summer car that requires premium fuel and gets meciocre gas mileage. You've robbed it of its reason for existence. This car is a blast to drive, but only if you focus on the driving, not the cost. Like most cars in its class, it's not a practical vehicle.

In answer to your question, I predict you'll get 23 - 25 mpg driving it like your grandmother in the conditions you describe. I have found Nissan's mileage estimates to be reasonably accurate. IMHO, be more worried about tire costs. They're $800 - $1200 a set and will get about 15,000 miles.