When I bought my Miata the dealer told me to use premium gas, but my wife recently suggested I try regular. At which point I considered midgrade gas, and noticed: one is better off mixing regular and premium than buying midgrade! For regular, midgrade, premium, the $/gal. prices were 1.77, 1.92, 2.02 , while the octane ratings were 87, 89, 93. So the first jump gives you 2 octane points for $0.15, while the second jump gives you 4 octane points for only $0.10. Since mixing gas averages the octane ratings, if you mix 2/3 regular with 1/3 premium, you make your own midgrade gas for only 1.85, saving 0.07.
Source: josephweisenthal
