This octane subject has been discussed at length at a VW message board I go to for my Passat. Yes a car that is rated for 91 can run on 89. The problem is that the engine's computer has to compensate for the lower octane and retard the timing, hense your performance and fuel economy will suffer. So you loose any savings you got from buying lower priced gas. My Passat has the 1.8, 20 valve, turbo engine. It requires at least 91 octane. Generally high reving engines require high octane gas. Hondas require only 87 or 89, but the Acuras require 91. Premium gas is a "cooler" gas in that it's design to burn at higher temps. vs. 87 oct. Running premium gas in car that calls for 87 oct. is generally a waste and may cause more deposits in the engine. Cars desinged to run on 87 oct ignite the spark plugs early, so premium gas does get burned completely. Now, like someone has already mentioned, running premium gas in a car that calls for reg. gas, on a long highway trip, will give some benefits. This would again be due to the fact that the engine is running hotter where premium gas works best, so you'll get better fuel economy.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section