My 2003 HCH has had fairly consistant mileage until recently. However, I've noticed that my FE has dropped from 43 to about 40. My car is 4 years old next month and has 99k miles. I'm wondering which factor is causing the drop in fuel efficiency:

-- The fact that ethanol has replaced MBTE
-- Are the batteries wearing out?
-- Or is it just in need of a tune-up?

Technically, it shouldn't need the mega tune-up for another 6k miles, but I'd do it early if that's the problem.

Any ideas?

Well, technically you should see about a 3% drop in mpg with ethanol over regular. I don't know about regular with MTBE added. 3% of 43 mpg is a little over 1 mpg.

I've been running regular instead of E10 the last couple of tanks to see if I can notice the mileage difference. So far, I can't tell the difference, but there are so many variables involved that I doubt I'll be able to correlate a difference.

Sounds like you need a slightly early tune-up.

Considering how few years it took to get to 99K, that could be part of the issue.

Best,

Michael