I regularly drive the Prius to Myrtle beach loaded with the wife, two kids, a dog and our gear. I drive 70-75 and average 49 MPG for the round trip. But that is withs some lower speed driving thru towns. My guess is 45 MPG at 75 MPH. SOunds like something you should consider if you can't slow down. Ir the Insight if you can get by with 2 seats.
Of course another thing to keep in mind in the gas versis tdi decision is the usually higher cost of diesel fuel, which would increase your operating costs even if the tdi did get somewhat better mileage than the gas car.
2002 WRX, 24 mpg (best was 29, but while driving through Indiana & Ohio, all hwy)
1989 Acura Integra, 28 mpg (best was 37, 10 years ago, driving to Fl, all hwy)
1994 Mazda Miata, 27 mpg
1995 BMW M3, 24 mpg (best was 29, driving all hwy)
2004 Honda Accord V6 (wifes car), 20 mpg (best was 32 all hwy)
My driving is typically 75% hwy, 25% city driving. This is all accomplished at speeds up to 75mph.
I know I should slow down, but I am just keeping up with the traffic. I would like to get a car that would get me into the high 30's while driving 75 mph on the hwy. I drive approx 30,000 miles a year.
I am tired of this kind of fuel economy, and am open to buy another car that would acccomplish this.
Your last quote above ( "Tired of...") kinda sums up everyone's feelings...especially at today's obscene gas prices!
Hint; Go to the "For Sale" section of this website... and click on "Another gas additive?". This will give you my mailing address.... ( note to admin., I'm not pushing anything!) Ciao!