very very poor milage

<div class='quotetop'>QUOTE(fearless @ Aug 7 2007, 02:22 PM) [snapback]491962[/snapback]</div>

That's not a good sign. Is it possible that the heat is coming from the HV battery behind the rear seat?
How hot is "extreme heat"?

 

What gear do you drive in: D or B?

Do you keep the cabin hot or do you use the A/C? If so, what's it set at?

Anything over 87 octane fuel is a complete waste at low altitudes.

 

<div class='quotetop'>QUOTE(efusco @ Aug 7 2007, 02:56 PM) [snapback]491988[/snapback]</div>

Very true. If the heat is about as hot as a heater, there's a real possibility that hot air is blowing through the vents under the front seats and come up toward the rear passenger. In that case I would also check whether the A/C or Fan is set to the Feet position.

 

Have you tried using cruise control on the highways? Try setting cruise control to 70 and drive with that and report back the round-trip mpg.

 

<div class='quotetop'>QUOTE(fearless @ Aug 7 2007, 03:13 PM) [snapback]491996[/snapback]</div>

Can you remove the rear seat to doublecheck? It's very easy to do.
There are two hooks in the middle of each seat section. Pull straight up and it'll pop out.

 

My guess is that your poor mpg arises from a combination of issues. I would recommend to drive at a constant speed on the highway using cruise control.

If you can do 50 miles on the highway at 65mph using CC when driving with no load and get sub-45mpg my guess is that there is an issue with your car.

I don't know that highway mileage would be affected by anything else other than a combination of severe head wind, heavy cargo or mid-30psi tire pressure. We are obviously assuming you are running at D (not at B gear) and that your traffic is not stop and go.

I have never managed less than 42 mpg and that was when the car and the tires were brand new.

 

Hi Fearless,

Check that your AC control temp is down below 83 F. Above I think its 85, the car automatically goes into max hot. This is just like max cold, but the electic heater and car heater both chime in to blow hot air. Its a bit of annoyance, just as the max cold is on cool days when you do not want heat, because one is in the winter coat, and inside the car is too warm for the coat.

Your oil level must also be BELOW the top pip on the oil stick. Three quarts of oil, after filling the filter before putting the filter on the car, results in 1/4 inch below the top pip. So that is like 3 1/4 quarts of oil MAX.

We are all assuming your new tires are 185/15 s which is the standard size for the standard Prius (not the Touring), as you have not mentioned changing wheels or tire sizes. What make and model are those new tires ? 17 inch wheels with 225 section width tires would certainly result in low 40's mpg.

The side-by-side test is sufficient for me to believe there is something to be figured out with your car.

Higher octane automotive fuels have poorer energy content per gallon. Use regular. Might try some injector cleaner with a tank. A half-plogged injector could be messing things up.

Being in a more rural area, you really need to check for rodent nesting materials blocking the battery ventilation exhaust under the wheel wells. There are black plastic tubes that run from the battery to rubber flaps at the wheel wells. This is where the air from the passenger compartment battery vent goes. Rodent nesting materials can stop the batttery cooling fan too. These can be accessed from inside the rear of the car by removing everything. A good site with lots of pictures is Hobbit's. Just google Hobbit and Prius, and you will find it ! I am sure he has pictures of how to dismantle the battery ventilation stuff on there somewhere. Do not dismantle the battery. You may find a trail leading into the battery. You will need a Toyota Tech to do that! There are a few cases where the plastic grills had to be replaced/reinforced with hardware cloth, and moth balls deposited in the vents. Mice nests have been found all the way inside the battery case! Being next to broad fields, the natural habitat of mice and volls, this is my best guess as to what it happening with your car. If this is it, donate some money to the local raptor rehibilitation group. A nest of red tail hawks in your neighbor hood will do a good job of keeping those critters under control.