Two weeks ago, I switched from 89 to 87 octane. I was averaging 19mpg highway on 89. Now I'm getting 21mpg highway on 87.

I was putting in 89 for about 1.5 months and I've never gone over 19mpg highway until I put in 87.
With city driving, I was averaging 10-11mpg on 89. Now I'm at 14-15mpg on 87.
I will be going out of town again this coming weekend and will be running on 87 octane. I want to confirm that I'm now getting 21mpg with 87 on the highway.
I currently have 1400 something miles on the odometer.