99 CRV, reduced gas mileage after timing belt replacement

I am the original owner of a 99 CRV, 2 wheel drive. I have always monitored the gas mileage closely - I mentally calculate it just about every time I fill the tank - and it's always averaged about 25 mpg.
The mileage is about 112,000
About 6 weeks ago, I had the timing belt, water pump, and drive belts replaced by a Honda dealership. They also changed the oil, replaced the spark plugs, and the left engine mount, which they found was broken
when doing the timing belt work. Within the same week, I replaced the tires (Costco, Michelin X Radial DTs) and had the front end aligned by the dealership. I replaced the air cleaner myself.
Ever since this work was done I'm averaging 22 mpg. Nothing has changed about my driving patterns, or where I get gasoline. I've contacted the dealership and they've told me that it is not possible that the work they did caused the gas mileage reduction. (I consider this to be a reputable shop, I've had work done there several times over many years) The advisor also implied that this really wasn't anything to be concerned about, and there isn't anything that they can "check" to find the cause. As I told him, I disagree, this is >10% reduction in gas mileage which is going to cost me about $250 / year.
So I'm hoping an expert can advise, is it really just a coincidence that my gas mileage dropped after this work?
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TOP POST
Are the tires the same size as the previous ones? Perhaps the O2 sensor could need replacing. I don't know if the tension on the new belts would make a difference.
DelCRVOwner wrote:

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Alan wrote:

Well my first thought was to check the engine's timing, both mechanical timing and ignition timing. However, it could also be related to the tires if they're sized differently from your original ones or are under inflated.
Eric
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