A few fuel economy questions

First, the octane number should make NO difference.....but the ethanol content WILL, regardless of the octane that results.
The general rule of thumb is that E10 can have a 3% negative impact, so I would expect E15 to be 4 to 4.5 %, versus E0 that is.

Your best mileage is likely to occur when driving in the 45 to 50 MPH range without any stops or a lot of speed changes.
That is regardless of what kind of vehicle you are in. But that isn't always possible for long stretches.

I think your expectations might be a little too optimistic here.
A 15 year old vehicle.....with how many hundred thousand miles on it......should not be expected to perform like new.
That is especially true if it still has the original hybrid battery in it.

 

I doubt that would be detectable.

On the other hand, there will be a detectable difference between E0/E10; you usually get about 3% better on E0 though it never actually pays for itself.

You're still getting pretty good numbers for a 15 year old Prius, assuming original battery, wheel bearings etc.

Most of the things you could do to restore it to "like new" performance will never pay off given expected remaining service life of the car and current low fuel prices.

 

Yeah those don't lose efficiency until they lose compression, and usually by then the whole car is beat.

The Prius will gradually lose fuel economy to a diminished hybrid battery, and that happens with simple age whether you use it or not.

 

Please give us an idea how short your normal cold-startup trips are. The 8 stop 60 mile trip results will depend strongly on how much the engine cools during stop layovers. Hour long stops will drag it down much more than 5 minute stops.

There is no particular test speed to try out. Drive at whatever speed is reasonable for the situation.

From past tests by PriusChat members and their associates, Prii get their best MPG at about 15 mph according to one group, or in the 10-20 mph range according to another group. But that is unreasonably slow for most driving needs and most roads, even dangerous and illegal. So the next best answer is that your best MPG speed is the slowest speed that is otherwise reasonable for your particular situation and needs.

Below is an MPG vs speed chart for a Gen3 Prius under ideal conditions, including a fully warmed engine that you don't get in short trips. (Bob reset his MPG meter after full warmup.) Subtract about 5 mpg for having a Gen2, then make more downward adjustments for winter cold season, E15 fuel, non-LRR tires, aging battery, any wet roads, etc. But you should still see the pattern of lower speed producing higher MPG all the way down to slow residential street or school zone speeds.

 

Two things use the most energy in a vehicle:
Starting out from a dead stop.
Bucking the wind resistance above about 10 MPH.
The trip you describe has the worst of both things, most likely......and with a cold engine.
Given that kind of a trip, you will continue to be disappointed and probably would have a similar disappointment with a brand new one too.

 

Stop signs, traffic lights, and congestion during a single trip segment don't really count as 'stops'. Those count as ordinary 'city traffic'. The stops that I would count are the ones where you completely shut off the car and allow the engine to significantly cool down.

That will be a much better gauge of what MPG your car can currently do, though the 70 mph portion will necessarily get lower MPG than the 55 mph portion.

I have a crude rule of thumb, from a previous car, that a cold start costs 2 miles of fuel. This rule sort of works with my Prius too. I.e. your 6.7 mile commute from a cold start needs as much fuel as 8.7 miles of warmed-up driving on a long trip (my customary long trip is 400 miles). This rule would suggest that if it gets 35 mpg on those commutes, then it should could get closer to 44 mpg on that longer trip, if you keep down to 60 mph. The faster section will drop things a bit. But remember that this rule is far from perfect.

For that longer trip, do reset your mpg display at the start (I forget, can a Gen2 do this?), and let us know what mpg you get at the end.

 

FWIW, I find using cruise control helps mpg since if I am not using it I have a temptation to overuse the accelerator. I use cruise control everywhere above 25mph. Maybe try it and see if it helps you.