Questions: Fuel Used 87 VS 89 oct -AND- MPG

I have been using 89 octane fuel in my '05 Double Cab TRD 4wd because the man that I bought it from always used 89 octane. I have a little over 61k on the truck and was wondering if it would hurt anything to switch from the 89 octane to 87 octane?

Also, I have kept up with my fuel consumption religiously and have been getting about 18.251 MPG over the past 10k miles. I do 80% of my driving on the interstate at ~75 mph. What gas mileage have you all been getting?

THANKS in Advance!

Matt
2005 Toyota Tacoma Double Cab TRD Sport 4wd
1999 Lexus LX 470
1979 Toyota Land Cruiser FJ-40

 

Would it hurt the engine to try the 91 or 93 octane for a few tanks for observation and then go back to 89 octane if not pleased? If it wouldn't hurt, I may try making the change for a thousand miles or so and see if I get better gas mileage and performance.

Matt
2005 Toyota Tacoma Double Cab TRD Sport 4wd
1999 Lexus LX 470
1979 Toyota Land Cruiser FJ-40

 

I use 87 octane & Lucas gas treatment in my V-6...E/G runs quiet, smooth & constantly gets 20 mpg. When we test E/Gs here @ Toyota, we use 87 for audits & 93 for durabilities.