The honda civic does indeed have a couple times more horsepower than many of the gas-miser cars of decades ago. And it does indeed get about the same gas-mileage. I don't see that as a good thing.
I find it a huge shame that people, left to their own devices, buy cars that have far more ability to accelerate than necessary. You can't buy cars with engines that weak any more, but if current technology were used on cars with weaker engines we'd get great gas mileage. Slow-accelerating cars don't sell because people associate them with a lack of status and a lack of fun. I don't think looking rich or fun is a justifiable reason for buying a car that pollutes more.
I drive occasionally and when I do I drive a decade-old toyota corolla 3-speed. When I'm not trying to merge on to a freeway (which involves quick uphill acceleration, since freeways are usually raised) I probably use no more than a quarter of its maximum power output, flat ground or uphill. (This saves a bit of gas.) When i'm trying to merge on to a freeway, I probably use no more than half of its maximum power output.
All in all, I'd be thrilled if I could trade the car in for a car with half the horsepower (and a transmission designed for 2/3 of the horsepower, because that transmission is a weak link.) I'm sure the reduction in weight, (and in weight rotating at thousands of rpms) would greatly increase gas mileage. If everybody learned to drive as though fast acceleration were out of the question, they could all get better gas mileage and lower CO2 output. We wouldn't get where we're going more than a minute or two later! Truckers get by with slow, slow acceleration and the rest of us could too, if we wanted to.