Hi, i just wonder what are the different octane number gazolines you have in USA ? I know that the US octane number is different than french (RON) number and seems to be (RON + MON) / 2, so a 98 RON (= 88 MON) would be a 93 . At this time i run my '57 Imp with unleaded 98 RON (so 93 for you) and i want to switch to 95 RON ( = 90 US gazoline, don't know if it exists..). My compression ratio is 9.25/1 so what is your advises ? I've found a table in the middle of http://www.faqs.org/faqs/autos/gasoline-faq/part3/ (paragraph 7-2) but i don't know if the "octane number requirement" means US or another octane number. -- Philippe COURANT (Pau, France)- Webmaster des sites ACCF et C-I-F Imperial 1957 Crown convertible Buick 1996 Roadmaster wagon - American Car Club de France (ACCF) : http://www.accf.com - Chrysler Imperial France (C-I-F) : http://www.ifrance.com/c-i-f - Cadillac " Standard of Excellence " : http://www.ifrance.com/accf-cad - SportsCars : http://www.ifrance.com/accf-sprtcar