Umm... it turns out that the car I used was not the 7, but the GT-ONE. Oops, I guess I should have read the thread title a little slower.
However, I had the same problem as you with the Escudo. No matter what I did with the aero or gearing, the thing basically stopped accelerating around 180 MPH. It might be a combination of the engine not producing any more power at that RPM and the aerodynamics. Or it's a bug. Or both.