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 TRí 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 can't be confused with regular tyres that won't fit these rims, although the system itself often causes confusion. Here's 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 Ví is the speed rating (149mph max) and the ëRí stands for radial. Finally, 340 denotes the rim diameter in mm. The TRX range features nine sizes. This 200/60VR390 fits the BMW 518, 520, 525, 528, 728, Mercedes Benz 200, 220, 240 and 300.