mixing for octane (LAC)

[William Magliocco]

This may appear to be off-list, but it's really not. 
Here's my question:

Due to the highway robbery at your nearest gas pump, I
am wondering if I can mix gasoline to my own octane
needs.  In SmogLanta, most stations only carry 87, 89
& 93 octane gas.  My '93 100 is rated for 91 Octane
gas.  All octane figures are the (R+M)/2 variety to
keep it straight.

Let's say I mix 5 gallons of 87 octane gas with 5
gallons of 93 octane gas.  Do I wind up with 10
gallons of 90 octane gas at the end?  If not, is there
a formula to figure this out?

I understand that gas stations that sell 3 grades of
gas generally have two grades of gas in the
underground tanks.  The pump mixes them.  

Anyone remember the 5 grade Sunoco pumps-they only had
2 grades of gas feeding the pump, right?  What were
the mixing ratios?  I also recall they sold a 94
octane on the premium side, and an 86 octane as a
"economy" regular.

Someone will know this answer...and no flaming
puleeze!

Required Audi Content-I mixed 2.5 gals. of 89 Octane
with 2.5 gals. of 93 Octane gas yesterday in my '93
100.  So far, so good.  Previous test with 5 gals of
regular a month ago gave me a lousy running engine.

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