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Hello - First post and 2003 Honda Odyssey may be a little out of place here.

I've always tracked and spreadsheeted all my fuel economy for all my vehicles (since 1985) so I trust my data.

Problem is that in/about March 2009 I experienced and continue to suffer a decrease in FE for my Ody van of about 8-10%.

Unfortunately, last March (2009) I changed several things in my 2003 Honda Odyssey at around the same time, so I cannot decide for sure where to start.

New tires. Changed the OEM Michelin Symmetry for Michelin Primacy - same size.
Had original set of spark plugs changed (at about 70K miles - by Honda dealer).
Had one of the six "coil-packs" replaced as van was throwing a code.
And lastly had EGR valve replaced under recall warranty and dealer also "cleaned ports" at no charge.

After this work was peformed the van runs and feels as good as ever but it was immediately apparent that my mileage suffered and for the balance of the year (again comparing to previous years) my aggregate mileage (since the changes) has been about 8-10% down.

I might describe the feeling I get as that the van is "running rich".

Previous to the reduction, I was very happy with the efficiency of the van - getting upwards of 28.5 mpg(us) on highway trips and averaging around 21 mpg(us) all combined.

Ideas?

An Odyssey is welcome here. ;)

Hmmm... None of those should make that big a difference. The Primacy tires are supposed to be decent low-rolling-resistance ones, although maybe not as good as the symmetrys. Worst case, I'd expect 2-3% lower when they're new, and gradually trending back to normal as the break in (which they should have by now).

I wonder if another of the coil packs is going bad, but not throwing a code yet. If it's misfiring and not burning some fuel, that would use more fuel for the same power. It's a big engine, so a slight power loss would not be hugely noticeable.

My knowledge isn't odyssey specific, however along with the newer tires, you mentioned having the egr replaced. This could be the culprit as this affects your A/F ratio quite a bit. This could be causing you to run rich and lowering your mileage. It might not show a code if it's within a certain range (anywhere from 12:1 up), but that would definitely cause the decrease in gas mileage you're experiencing.

It wouldn't be harmful to go back there and explain the issue and what you think it may be to give them some idea of where to look.

I changed our '99 Odyssey's 2 Oxygen sensors at 95,000 and the MPG's increased by about 10%. The MPG decline prior to the change was steady though over time.

Thank you for the suggestions.

I have a ScangaugeII but have not been using it lately - but even when I was, no codes were showing.

My thoughts were also on the EGR or perhaps spark-plug gap. I had not thought of Oxygen sensors.

Seems another trip to the dealer is in order - but that's a shame because the lost mileage is costing me about $150 for the year so either way I pay for the fuel or for the "diagnosis".