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Timing belt replacement

Can replacing a timing belt change your gas mileage e.g. form 21 mpg down to 18 mpg?

Yes, and for one of 2 reasons while speaking in general terms.
If the cam timing is off a tooth or two.

If (for distributor equipped cars) the belt timing is correct and the distributor controlled ignition timing is not due to improper dist. installation or an improperly timed balance shaft. (if the balance shaft drives the dist.)
(In theory, if everything is reassembled into the same position the ignition timing should not be off. In practice it can be and that is why the timing should always be checked with a timing light no matter what. Unfortunately, many shops/mechanics simply assume that it’s correct and don’t check it.)

Hope that helps.

This sounds like you recently paid to have your timing belt changed. And now your gas mileage has fallen from 21 to 18 mpg.

If the timing belt was reinstalled “off a tooth” it would likely affect the gas mileage. However, you also should look for other symptoms like vibration and not idling smoothly before declaring the belt is “off a tooth”.

Can you fill us in on the “rest of the story”?

Good point about the ignition timing on distributor controlled cars. I had forgotten about that in my reply.