Consistently getting 35-37 mpg, nothing higher

If your engine, after it has warmed up completely, runs when you come to a complete stop, and the battery is full, then something is not right. Normally the engine shuts off right away at a stop, or even before as you slow below 8 mph.

If the battery is full, and you are descending a long hill, then the engine will eventually go into "B"mode, which is engine braking as in a normal car. You can also select this on the shift lever, which is a good idea. It does not "get rid of excess charge", it just keeps the charge current from being generated in the first place by taking the load off of the regen and friction braking systems. But this only happens at speed.

If you find the battery gauge quickly goes through large swings from empty to full with normal driving then you might have HV battery problems, perhaps a bad cell or two. Using a scanner and software would allow you to read out the various module voltages and see how much variation there is.

 

That is incorrect. There are several circumstances that can cause engine to continue to spin after stopping, and top on the list is that exact thing that the OP mentions, excess HV battery charge.

Anytime you have to stop with full 8 bars of SOC then this can potentially occur. (As well as the opposite situation of course, where the engine can continue to run if the HV battery charge is very low.)

Another circumstance is the need for cabin heat, though as you say, if the engine is completely warmed up then it should be able to provide heat for some time without running the engine. Never the less, when diagnosing an "engine running when stopped" issue I'd say you should always turn off all heat to be sure that this isn't the cause. Turning off all heat on the gen2 prius isn't as easy as you might think. Unless you set the temperature all the way down to "max cold" then even if both AC and auto-AC are completely off the heat will continue to run, even if just the fan is blowing.

The other possible cause is "hybrid stage 3a" which will cause the engine to run for up to 10 seconds after stopping.

 

Don't baby the throttle too much. When you need to accelerate then it's almost always best to use the gas engine for that purpose (and hence save the battery for where it can have maximum effect, where you can glide).

Acceleration is best done at a medium throttle, not flat out, but not too light either.

 

I just bought a 2012 Prius 3 with 3 miles on it. Funny enough I was getting about 40 on the Eco Mode and about 46 in regular mode. Not sure if the 2004s have the different mode but maybe u can try that. Could it also be that with all the miles on it it's performance has just gone down?

 

The Gen2 does not have different modes. Performance doesn't "just go down", there's always a reason or two but it can be a bugger to find.

 

Make sure the optima is fully charged before installing.
I have a gen2 and once it got mileage in the low 30's, tracked it down to a misfiring spark plug, didn't set off any alarms but definitely affected mileage, good luck with the dealer, get an estimate and think about the services they offer, even post it here, some dealers are great some aren't.

 

Are there any Prius specialists like Luscious Garage in LA?

 

I would try a set of LRR tires (Energy Saver A/S or Ecopia EP422), cleaning/adjusting the rear drum brakes, alignment and replacing the 12v battery.

 

Automotive Services | Modesti's Car Care Center