Loss in MPG
 

I have a 2016 Mazda 3 I have noticed overtime my MPG has reduced significantly. When I used to fill up my tank my range depending on how much city/highway I was driving would show somewhere in the mid 400s to sometimes in the very low 500s. Now I get mid 300s. Driving to work I would average MPG in the 40s to 50s one day I even managed to get 60 (by driving extremely conservative just to see) but it would usually be mid 40s. city I would get about 30. Now I seem to get low 30 on highway and 18-20s on city. I have new tires (same brand and model) and a clean air filter. What else could be causing this? I have around 80,000-90,000 miles on it.

Spark Plugs can more directly effect the MPG then most other parts or operating perimeters of an engine? If you have more then 20k miles on them they are over due to be changed. What so many don't realize is spark plug will wear faster in a linear rate as more miles are accumulated on the engine.

Pretty amazing reported MPG.................. the % interestingly greater then advertised and no apparent upgrades to increase the average MPG?

No upgrades at all. I ordered spark plugs the other day waiting for them to arrive. Maybe I'm noticing a bigger drop because I was getting such great MPG. My dealer offers a fuel induction cleaning service for around 100. They say its not just adding chemicals either, is that worth it or a waste if money?

Depends on really what they are actually doing regarding a Fuel injection cleaning?
The cost is right in line with a Fuel injection cleaning service. The thing is normally the Service Machine is connected to the F/I rail and a cleaner is introduced under high pressure?

I believe thats what it is. Is it worth it with around 100,000 miles or is it just an easy service for them to make money on?

Its hard to say, why did they say you needed it? About the only time I recommend it with customers is if I first diagnosis idle, off idle and poor overall throttle response without a DTC as possible dirty Fuel injector(s)? Other test to confirm would have been using oscilloscope but that would have been a basic hour itself. However it also would have given a more complete DATA of information on many aspects of the engine while running.

Find someone who is experienced with a scan tool & can read & interpret live data. Get the activity of both the A/F & O2 sensors analysed.
A lazy A/F can make a huge difference in fuel economy and won't create a fault code.