When I got my '95 savage, with 9k miles on it,  it got about 50 mpg. Now, with 16k miles on it, its getting 45 mpg.

What are some possible reasons for this drop? Are there any tuning or modifications I can do?

There are some things I have done that may have affected my mileage. I have adjusted the idle mixture, white spacer mod, and added saddlebags.

Or is the drop in mileage simply due to my rear tire shrinking from use? It was new when I bought the bike and is now almost ready to be replaced.

Also for those who have upgraded intake and exhaust and rejetted, did you experience any change in mileage?

Thanks.

I would venture to guess that the likely cause for the mileage drop is a change in riding habits.   As you get more comfortable on the Savage you are tending to twist the throttle just a wee bit more????

Should we factor in cam timing change due to cam chain wear?
I was thinking it was a trade off, MPG for SPM,, MPG, you know,SPM = Smiles per Mile,  The Smile Injector grip, twists more coming out of turns, The BEST fun is being leaned over & bringing the throttle in just enough to feel the thing slip just a bit.

 I would say bald tire will gear it lower,white spacer makes it use more gas when you take it out or make it smaller,saddle bags wind resistances.

ok, in my experience with car motors, as they age they develop carbon depisits on valves and pistons therefore inhibiting efficiency. also, I concur with replacing the plug and checking the entire ignition system for proper working order. next, a new air filter, preferably a freer flowing one will improve efficiency. the tire is a definite possibility, I would replace it soon and one more thing, on each vehicle I own, roughly every other oil change I run sea foam through my engines. I would imagine that oon the 650, you would remove the intake hose and pour slowly into the carb of a running motor until it dies. then, wait about a half hour then start it up(it will be tough). when it starts you will want to rev it up and hold it at a level roughly 4k for how long it takes for the white smoke to quit rolling from the pipe. sea foam cleans off the carbon deposits in the head and in pistons.

this list is what I would do.

I don't think you need to worry about state of your engine,
doing spacer mod reduces the backfire problem because it makes the  needle sit higher in it's slide. As the bike comes, (with the famous white washer and the idle screw set lean), you get almost no petrol through when you close the throttle.
You have increased the amount of fuel your engine burns so you will get an instant drop in MPG. The bike should pull from low revs better, idle stronger, and not backfire so much on overrun.

Also saddle bags are a bit like putting a roof rack on a car, creates more drag which = less MPG.

One thing you may not have looked at, Air Filter?
If it is starting to restrict airflow it will make bike run richer and burn more fuel. The standard Air filter should be replaced every 7,500 miles, (more often if used in a very dusty climate).