Although it was very difficult to drive without proper tuning,
the Dodge Concept Car (Copperhead) was the fastest car in GT1
with full upgrades...not necessarily top speed, but it did set a majority
of my lap records in GT1.
In GT2, I would have to say the Escudo was by far the fastest, but
most of my lap records in GT2 were set with the ZZII. The Lotus GT1,
GT-One, and R390 were all very close.
In GT3, obviuosly my records are in F1 cars, usually the F 094/s...
for convential cars, the GT-One, R390, 787B, Zonda, and ZZII, are
fastest for me.
GT:C and GT4

...don't know, haven't played either.
My fastest speed ever was with the GT_One on the Test Course in GT3.
Mach 2.5
yes that's right, 1800mph.
This is no lie.
Test Course lap record : 1:03
ask me how...