Hey Jeff.  How's it going?  Fortunately I didn't damage the motor; it just didn't want to run.  (It sounded and smelled great warming up in the trailer, but that's about it). 

If your buddy has a stock sled that he's going to use for recreational riding, your proposed fuel mixture is not going to work.  They are programmed at the factory to run on 91 or 92 pump gas.  If you try to run anything more, you'll notice a power loss and it won't RPM up. 

I'm not sure what the ratio is, but we used to mix 15 ga's pump with 5 ga's race for a variety of things. Boats, dirt bikes etc. I do know that an engine only needs the octane it can use, and the rest is a waist. Hos' is right (I know, sounds strange) 115 will make a 91 engine run worse. The higher the octane the slower the fuel burns, thus it's need for high compression engines.

And if you listen to any of that garble, you'll be burnin' some aluminum soon.

Thanks Joey

Not sure if this was answered or not, but if you are mixing two unleaded fuels or two leaded fuels, then you can mix a gallon of 100 octane and a gallon of 90 octane and end up with two gallons of 95 octane.

If you mix a leaded fuel and an unleaded fuel then the equation doesnt work.

Is is true that an octane number is based on RPM.  Octane is a knock rating.  To get the final octane (like what you see at a gas station), the fuel is run through a motor at 600 rpm and the engine knock is quantified.  This is the RON.  Then the fuel is run through the motor at 900 rpm and the engine knock is quantified.  This is the MON. 

The RON is added to the MON and divided by 2.  Giving the final octane rating.  This is why you see (R+M)/2 printed on the gas pumps.

There you have it, a REAL explaination.

Thanks for the input!!