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Drop in mileage for Honda

I have 2001 Honda Accord EX, 4 cylinder. I have been tracking the mileage for over a year and was consistently getting between 28-32 MPG - pretty good! In August it dropped precipitously to between 23 and 25. @ months prior I had the timing belt changed and a month prior I had the air filter changed. I took it to the dealer but because the Check Engine light was not coming on they didn’t want to just guess. The adjusted the valves and changed the spark plugs - no change. My Chevy pickup has had no change in the mileage so I don’t think it is the gas. The engine does slip occasionally but the Honda dealer thought the transmission was fine - they couldn’t get it to slip. I have another mechanic who I trust that I want to look at it but thought maybe I could get some ideas for them to start with.

If it was fine before the timing belt change I would suspect that the valve timing is off by one tooth.

I just checked my calendar and I had the timing belt changed in March and the mileage didn’t drop until August.

Round up the usual suspects:

Tire pressure, wheel alignment, brake drag, stuck engine thermostat.

If your Honda has a cable that goes from the throttle to a bell crank on the side of the transmission, check that this cable does not have any slack in it. If there is slack, it must be tightened until it is just taunt, no more than that.

Consider dropping by a local AutoZone, Advance Auto, Checkers, etc. and have them pull the codes; which can exist even with the CEL off. Post any results back here.
They will do this for you free and it only takes a few minutes.

If the problem surfaced after the timing belt change a couple of possibilities could be that someone is off a tooth somewhere on one of the timing sprockets; generally the camshaft. It can run like this but is not really “right”.
Another could be that during the timing belt change the distributor ignition timing was altered and was simply left “close enough”.

Even when the distributor is not moved at all the timing should always be inspected after a belt change as a couple of degrees variation in timing can affect mileage a lot.