Burn off a gallon of gas and go to the local paint store for a gallon of Toluene.
If you have 17 gallons of 87 and one gallon of Toluene, your 87 will increase to roughly 89 Octane.
Burn off 2 gallons and add 2 of Toluene to get you to 91 Octane.
These are rough figures, and are proven very safe in half a dozen cars here. Many of us have mixed well over the 95 Octane mark.
This was back when TWM still had the Crow, but you can see you're not the first. It'* a good read, especially when Mrs. TWM popped in for a visit: http://www.bonnevilleclub.com/forum/...hlight=toluene