I've read repeatedly on these forums that it takes about 5,000 miles of use to "break in" a hybrid and get optimal MPG. Is this real or a myth?

I think its pretty real as the engine, drivetrain etc. lose some friction.

At the same time, most new owners are on a learning curve on how to get high FE out of their new car, which may take a least several thousand miles to get pretty good at.

So, you are probably breaking in both the car and the new drivers habits.

The rule really applies to all cars, not just a hybrid. The electric engine really doesn't need a break in period where as the ICE does.

The HV Battery also tends to get better as it gets "broken in". You'll find that you're able to draw more from the battery as it gets older.

First 4WD Hypermiler

Have you read the FEH FAQ?

Live in Michigan? Let it be known in Michigan Roll Call

Read My Automotive Blog at Rich Rambles

Any links on the battery getting better with age?

Not off hand, but there have been discussions about this before, and there was also the battery engineers talking about it at the FEH Experience.

First 4WD Hypermiler

Have you read the FEH FAQ?

Live in Michigan? Let it be known in Michigan Roll Call

Read My Automotive Blog at Rich Rambles

This is reassuring to read because we are well below the 41 MPG Nissan advertisies the Altima hybrid to offer in city driving. our MPG meter says we are getting 36.4 MPG average, which I'm acutally pretty happy with given that we really like the car... but I'd love to see the numbers go up.

I would tend to believe until I see data otherwise, that your ICE breakin period will result in a higher percentage gain on overall mpg compared to what the battery portion will offer.

Actually the current EPA numbers for all cars is optimistic and do not
reflect the real average fuel economy. This is even more so for hybrids whose real average FE numbers are almost 10-20% lower than the EPA
combined FE for 2007.

Example (all hybrid models):

CAR EPA Combined AVG Actual AVG (Greenhybrid)
----- -------------------- -----------------

Prius 55 47
Camry 39 37
Civic 50 46
Highlander-4WD 29 25

So the EPA is changing its FE calculation method for 2008 to
make the numbers more realistic.

For 2008 the combined EPA numbers are as follows:

Prius - 46
Camry - 34
Civic - 42
Highlander 4WD - 26
Altima - 34

Hence the fuel economy you are getting is very near the real average
for normal driving. This does not mean that the current EPA numbers
are not achieveable but it will require very careful driving and adhereing
to many hypermiling strategies consistently.

Note also that the Camry and Altima have identical "estimates", so Altima owners shouldn't feel like second class hybriders. Further, the car manufacturers are not the ones saying the car will get that mileage. It's a mandated test from the fed. govt. This is not to say the car manufacturers don't use this data as selling points, because they do. Shamelessly.

A NiMH battery will increase in capacity for the first 5-10 charge cycles (if you charge it up fully). It will probably take a few more partial cycles. Whether this will result in improving fuel economy I can't say, but I don't really see how, as the computer runs the battery from 40% to 80% full charge, so it shouldn't see the increase in capacity. Perhaps internal resistance also drops, and that would result in improved mileage.

Pearl is a
2007 Driftwood Pearl Prius
Package "B" (everything but leather, nav, and rear camera)