According to the Suncor engineer, gas stations have two tanks, regular and premium. Mid grade is mixed at the pump.

On 2012-02-05, at 7:12 PM, Automatic digest processor wrote:

>  One difference, however, may be the consistency of
> mid-grade gas. In many cases the mid-grade is literally a mixture of regular
> and premium.  A recent expose here in PA claimed that in many cases the
> ratio of premium was insufficient to reach the claimed octane.