" Bias ply tires are much more flexible than radial tires, both in the sidewall and in the tread. This is why true drag slicks still use bias ply construction. You want them to flex and dig in on the launch.
They tend to have much less grip when turning, and less predictable at the limit handling characteristics than radial tires. So if you ran radial tires in the front with bias ply in the rear, in a turn, the front tires would tend to bite, while the rear would flex, up until the point they lost traction, resulting in unpredictable snap oversteer. "

" Actually, yeah, that's a good point. Running R compound tires in the cold and wet is generally a bad idea, even with rain grooves like the R888. "

" It's dangerous to mix bias-ply and radial tires due to vastly different handling characteristics.

It's generally safe but not optimal to mix different radial tires. It could result in unpredictable handling if you had sticky tires on one axle and harder tires on another, particularly if the hard tires are on the rear.

Since the R888's should be stickier than the NT555's, you'll probably see more tendency to understeer, but for street driving shouldn't be unsafe. "