I've known several people with Hybrids, and none of them have gotten over the mid-thirties in gas mileage. Maybe some people do, but I haven't talked to any of them. Also, the Honda Civic Hybrid compared to the regular Civic, requires around 200,000 miles to recover the initial price disparity, that's assuming the batteries don't have to be replaced. The VW Beetle Diesel I had got a legitimate 45 mpg and could cruise at over 100 mph (didn't get 45 mpg at that speed, but got 45 at 70 on the highway.) I think the diesels are being overlooked because they're too practical. True Greenies want some pie in the sky thing that nobody's ever thought of. For the Greens to accept something, it has to require massive initial cash infusions, use solar or wind power, take at least 40 years to recover initial investment, and have a life span of around fifteen years.
This Prius owner gets 51, or 55 mpg if I try hard.
The only time I ever get lower (maybe the mid-40s) is if I have 5 people in the car with the a/c on and I'm driving faster than usual.
My wife drives a hybrid 4wd SUV. She gets an honest 31 mpg, Her previous car was a similar SUV that got 14 mpg. Based on the miles she drives, the gasoline savings are making her car payment.
As far a diesels go, to get the high miliage smooth running euro engines, you need to use ultralow sulfur so the sensors work. Expect them after the full transition to low sulfur diesel.
Of course, I paid $4.49 for diesel yesterday which takes some of the economy away.