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Timing Belt & Poor Gas Milage?

I have a 2005 Honda Civic. In June, I had 100,000 mile maintenance performed on my car. With this maintenance, the timing belt was replaced, front engine seals and clamp replaced, oil changed, and wheels balanced and rotated. Since then, I have noticed that I have been getting poor gas milage (28mi/gal vs 33mi/gal). Is there any way this could be caused by the maintenance performed in June? If so, what needs to be corrected?

Yes, it’s possible. Some causes could be:
Inadvertent air leak due to a vacuum hose or fitting problem.
Belt off a tooth or two on the marks.
Ignition timing off.

In regards to the latter, your car is distributor equipped and has adjustable timing.
In theory, the timing should not change even during a properly done timing belt job.
In practice, the timing should always be checked and adjusted as necessary although I would wager that this is seldom ever done.

What would I do? Recheck the timing belt marks and ignition timing as a first step.

My 2002 Civic does not have a distributor and the timing is set by the ECU, I believe the 2005 is similar.

The two are extremely unlikely to be related. If your timing belt were installed incorrectly your engine would have been destroyed. When were the spark plugs and air filter last replaced? Also, I’d hope this gas mileage calculation is based on several tanks of fuel, not once or twice or even thrice.

I read that “adjustable timing on a 2005” and grimmaced.

That is not true if the belt was installed only one or two teeth off. The engine would not run very well though.

You’re correct, I’m wrong on the distributor thing. The car is an '05 and for some unknown reason I was thinking a 2000 model.

Was the proper weight of oil used?

Did the shop check and adjust your tire pressure while it was in? Maybe they got it wrong.