85, it depends alot on the vehicle. Higher octane fuel is better suited to a higher compression engine. Higher octane fuel actually is harder to burn than a lower grade fuel, since it is more "pure", but burns hotter once ignited. On a lower compression engine, it is harder to compress or squeeze a higher octane fuel mixture enough to get it to ignite properly. Thats why they used to recommend the use of 87 octane more frequently, since the compression ratios of the majority of the 80's engines were in the 7.5-8.5:1 ratio. Now that the manufacturers are back into a "more power" frame of mind, like they were back in the 60's and early 70's, they're once again designing engines with higher compression ratios which can take advantage of the higher octane fuel. Still, many of the newer cars recommend 87-89 octane over the higher 93 octane, depending on their compression ratio. "Premium fuel" is a hard thing to describe anymore too, as many stations offer premium fuel, which is only 89 octane, and not the 93 octane that many people are used to. I run 110 octane in my 72, which runs an extremely high compression ratio and needs that octane rating to run properly. 93 octane just will not cut it. However, it also costs me on average about $ 4 a gallon, but its the price ya pay to play...lol