My '00 Mustang GT is only getting 16.5 mpg. Mixed driving, going easy on it. Has an aftermarket 93 octane tune. Do these "tunes" do this?

That's a bit low, I had a 2003 Mustang GT (manual), when it was a bolt-on N/A car ( CAI, TB/Plenum, exhaust and a tune and 3.73 gears) it got around 18-20 MPG in typical driving. You car is rated for 15 MPG city and 23 MPG highway, so 16.5 MPG over is in ballpark, though If it's a manual, I'd expect closer to 18 MPG overall, if it's automatic, a little lower.

A 93 octane tune will just bump up the ignition timing/spark table. If you're using 93 octane fuel, you fuel economy should see a slight increase if anything.

Test the mileage on pure highway driving. Over 20 mpg and the consumption is normal.

If the weather is cold than that is normal .