Rick53
46/11 high gear with 700 X 45 tires is 20% higher than professional cyclists used until fairly recently. Is this the only gear you use? If you are averaging 12 mph it seems you would rarely use that gear for general riding.
The dead spot(s) you allude to are the drawback of wide range double drivetrains currently in vogue, particularly those with less than 12 rear cogs. Your average speed will most likely go UP if you get LOWER gearing with more evenly spaced gears in a range that is more useable for you. Your partner's triple drivetrain most likely has more useable gears in your 66 year old power range that you find comfortable. Shorter cranks can also help with this for someone with your inseam meaurement.
How do you know you are 1.4 mph faster on your friend's bike? And is your friend SLOWER on your bike? Same computer calibrated the same? Same tires? Are the saddle and handlebars identical and in the same relationships. A difference of a few millimeters in your riding position can make a surprising difference in power output.