Until the TRX, low profile meant a sidewall height that was typically seventy per cent of the width of the tyre. The TRX tyre is one half of a system that combines the road wheel with the tyre to spread the stresses created in a much lower-profile tyre designed for high speed work. The TR stands for Tension Repartie or distributed tension. The TRX, which runs in profiles all the way down to 45, uses a metric system of measurement. This means it cant be confused with regular tyres that wont fit these rims, although the system itself often causes confusion. Heres a quick primer: the 190/55VR340 has a 190mm section width (191mm, actually, but this would look clumsy on the sidewall), the profile is 55, meaning the sidewall height is 55 per cent of the width. The V is the speed rating (149mph max) and the R stands for radial. Finally, 340 denotes the rim diameter in mm. The TRX range features nine sizes. This 220/55VR390 fits the BMW 525, 528, 728, 535, 635 Csi, 735i, Ferrari 208, 308, and Mondial 3.2 (front).