I have had my 2004 G35 for about a year but I don't drive it that much because I ride a motorcycle in the summer time and my Jeep in the winter. I just put new tires on it Goodyear lS2 235/45 R18 not staggered all four are the same size so I could rotate them. I never really drove over 90 mph before because the old front right tire would get a slight wobble over 80mph and I mostly drive the car in town. However, with the new tires it is as smooth as could be at all speeds. The thing is the front end feels like it floats aerodynamically over 90mph and the struts and springs are new. I am wonder if this is common? Also, I got my car used and it has these huge red aftermarket sway bars on it, front and rear, is there a chance that these are causing this sensation over 90mph? I am not complaining because I rarely go that fast but I would assume the nose would have more of a planted feeling over 90mph like my BMW and Audi did.

 

I am also assuming the car should have splash shield under the front end but there is none, if that might make a difference

 

The splash shield helps with some aero, especially at higher speeds.

If you have 18 inch wheels, the widths should be 7.5 inches or 8 inches. I know that on the 19 inch rays, the front 8 inch width wheel takes 225/40/19. 235/45/18 would be wider than normal on 8 inches, and way too wide on 7.5 inches.

As far as I can tell, even with the matched widths your tire size should be 225/45/18 and 245/45/18. With the 235 width you have, your back end is going to feel tighter and the front end is going to be looser, which definitely is going to enhance the high-speed floatiness.

If anything, fatter swaybars will make the car sway side-to-side less, so those most likely don't contribute. I've got 18x8.5 and 18x9.5 on my coupe, 245/40/18 front and 265/40/18, lowered a few mm, aluminum splash guard (Tollbooth *****). Car feels pretty solid at 110mph on ok roads, so I'm pretty certain your tires, front especially, are the problem.

Fully square setups aren't super common on the coupes; the VDC can freak out if your rolling diameter is out of the 0-3% difference range. It seems to play nicer on the 3% end of the spectrum as opposed to the 0% end. The only time fully square is generally used is for dedicated track work. It's just too much of a hassle otherwise.

 

Honestly what you're feeling is just the 18" rubber compressing and smoothing the imperfections of the road at high speed, once you start actually pushing the corners hard at higher speed you will feel the car set and hold. The stock springs are a little progressive to take out some of the harshness you get from a spring typically set at those rates. If you have 80k+ on the OEM struts/shocks then you should just replace them because they're probably shot because you might be feeling squishy dampening, hard to say since I'm not in your car but mine smooths out considerably from 70-100 mph.

I think the car drives the best between 85-90 mph.

 

i had this same feeling when going over 85, seemed super floaty almost disconnected feeling. front strut bar really helped with high speed stability but also introduced in cabin noise/vibration and pedal vibration. so it was sort of a trade off for me

 

235/45/18 has a bit more sidewall for the front tires on this car compared to stock... that will absorb some shock.

Check your tire pressure too since you don't drive the car much (could be low from sitting)... the car will feel a bit unstable and floaty if the tire pressure is too low. My '04 feels great at all speeds as far as road feel. I'm surprised you notice it that much. I (like you) ride a motorcycle mostly during the summer and a 4x4 (2003 Tacoma) in the winter, so my G doesn't get a lot of use (literally like 1200 miles a year now). Whenever I get in my G35, it feels so different than everything else I drive. It's very low, planted, stable and the tranny and clutch feels super toy'ish and light... at least in comparison to my truck.