Was this an E9x M3? ... what an idiot
I could immediately tell the difference between 87 and 91 in a 328i rental car... it came with 87. After 1 tank of 91 it was much faster. When I put 87 back in it to return it as recommended by the agency, the car was noticeably slower.
M3 Sedan - AW/Black - 6MT - no moonroof - no folding seats - RAC RG63 - clear bra by Omega Werks