Well, low pressure turbos like in volvos and stuff can be very efficient, an I4 with the fuel economy of a 4 and the power of a 6 or whatever, but high pressure turbo, like in sports cars is just ridiculous gas milage.
and comparing the fuel economy of a skyline to an old muscle car is just ridiculous, for reasons such as carburetors and auto trannies with 3 gears and no overdrive gear(s). old, inefficiently tuned engines with crappy transmissions running on crappy gas.
You could compare the fuel economy of a skyline most fairly to the economy of a 6 speed F body, i'd say.
As for the Buick Grand National...
The Grand National was made in the mid 80s (83-87 i believe 83 didn't have a turbo, though). Identical to the Buick Regal, the Grand National was a turbo Buick Regal with black paint. You could get a normal regal turbo, T type or turbo T in any other colour, or a luxury regal limited with a turbo... to sum it up, you could get pretty much any 1984-1987 Buick Regal (and a few other buicks) with a 3.8L single turbo (intercooled in 86 and 87) V6. You could also get the turbo engine in the FWD Buick Riviera luxury cruiser.
Buick started playing with turbocharging in the late 70s as a way to get their V6 engines to make V8 power with V6 fuel economy. The early carbed turbo engines of the late 70s and early 80s made nearly 180hp and had horrible turbo lag and detonation problems, but they were a start. in the late 70s/early 80s in buicks lineup, you could get a turbo in pretty much any midsize to large car (regal, lesabre, park avenue, riviera).
The G body platform itself is an 'oldie but a goodie', 70s technology and chassis design, not too heavy, fairly rigid, full frame body on frame design, A arms in front with a solid rear axle. A midsize platform, which the following cars were based on from, not all at the same time, though.
1978-1988, A/G body platform (was originally the A body, switched to G body designation when A was taken over by the FWD celebrity/ciera in '82)
Chevrolet:
Monte Carlo/SS (2 door)
Malibu (2 door/4 door/wagon)
ElCamino (2 door pickup) (and GMC cabellero?)
Pontiac:
LeMans (4 door/2 door) Lemans Safari (wagon)
Bonneville (4 door)
Grand Prix (2 door)
Buick:
Regal/T Type/Turbo-T/Limited (2/4/wagon)
Century (4/wagon?/3 and 5 door hatch)
Grand National (2 door)
Oldsmobile:
Cutlass Supreme/Calais/Salon/442 (2/4/wagon/3 and 5 door hatch)
When you come down to it, the Grand National is a Turbocharged Personal Coupe based on a family car platform that has 16"s, fender flares, and looks kick ass cool. Isn't that basically what a GT-R is? a turbo 2 door family car.