Timing belt vs mpg?

[SSSVX]

Just got my timing belt and allll drive belts changed!
Question:
If your timing belt and drive belts are too tight or may
be the timing off a bit? Would these decrease the mpg?

But the car runs great!! The timing seems good.
No more belt whining noises from its first start!
The tensions also seem okay now.

I just really can't figure out why my mpg average drop like 2mpg.

Provided right after belts changed, I had a fresh oil changed, too.
Should have been better mpg with fresh oil.
The only thing I think of, I got 5w20 instead of 5w30 from last time.
Same brand, synthetic blend.
Thinner oil should have better fuel economy too.

Well, it's been like one thousand miles driving after belts changed
average dropping like 2mpg. why?
I know when I'm driving hard! But, I have like very consistent
mpg driving for more than one hundred thousands miles.
The testing after timing belt/drive belts changed are under:
same driving behaviors, same driving conditions, same brand
name of gas, same gas pump station at same outside temp,
of course same driver just me

I'm just a lil bit lost so far, why dropping.....
I usually got like 2mpg improved even after oil changed.