New owner, disappointed in mpg...

Winter tires. Ouch.
Cold temperatures. Ouch.
High speeds. Ouch.

I can't say what a Gen 2 can do at that speed, but you'll definitely get better performance in warmer temperatures.

What are your tire pressures? Even a few PSI above door can help significantly. I aim to ensure that the pressures are at least placard on a very cold morning and they'll be higher most of the time and in the evening.

Also, blocking your grille will help a bit, although given the distance you're traveling, more from aerodynamics than the warm-up. An engine block heater will also help, especially if your car is parked in a garage at night.

 

Well, snow tires and high speed - I don't know what you expect. Your mileage is pretty good for that combination.

Tom

 

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Your speed and temperature definitely hurt. A grill block may help keep the engine warmer in those chilly temps. Any icy or wet roads will also hurt. We don't know the type and pressure of your tires, or the condition of your 12V battery, which might hurt. And your 2008 won't do as well at these speeds as a 2010+. Under these conditions, it isn't obvious to me that anything is wrong, all cars will do poorer under these conditions than they will at 60 mph in nice dry spring conditions on summer tires.

 

Tell us more about the tires, model, size, air pressure.
78 MPH may possibly achieve 40 MPG at 75F.
Engine Block Heater and Grill Blocking may both help, I would own a scangauge II to monitor engine coolant temps.
If it is still the original 12 volt battery I would test it. Some failure modes lower MPG.
Coast up hills, speed up down hills.

At 78 MPH I would be making sure the aerodynamics are perfect, most of your energy is spent pushing air. 42/40 PSI air pressure in the tires, grill block to improve aerodynamics and engine warm up. Skinny tires. Wash the car.

 

What was the Park Ave. getting 13?

 

...another thread yesterday (Popular Mechanics) gave non-Prius numbers:

Impact of Speed (Popular Mechanics - FORD whatchacallit)
47.5 MPG @ 55 miles per hour
33.5 MPG @ 70 miles per hour

...you can see the math would be even uglier at 80 Miles per hour.
PM gives math equations. Prius is better due to design factors.

Let's face it Tricky Dick Nixon was right about 55 MPH speed limit when fuel savings actually had priority to the country.

Mileage Moment of Truth - We Put 40 Mpg Claims to the Test - Popular Mechanics

 

I easily get 43-ish with a carload of fandamily at 75 mph, with hills and freezing temps.

It's the tires.

 

I have Kumho Eco Solus KR22 at 40 psi. They are okay. There are certainly better LRR tires out there. But, I didn't have any problems last Friday when we had (unusual) 4" of very wet snow that froze to the pavement as a 1" layer of ice, much like what I remember from the Pacific Northwest. Usually we get champagne powder that can be swept away with a broom.

Take a look at F8L's list:

http://priuschat.com/forums/gen-iii...esistance-replacement-tires-current-list.html

 

Don't bother with measuring right after it is driven, because it will be temporarily better than normal. Instead, measure it first thing in the morning, before the ignition (the DC-DC that charges the 12V from the traction battery is what matters) is turned on. That is when the battery is at its weakest, and when you really need it to work.

In all my nonhybrids, 11.9V was effectively dead, though measurements were always above freezing. I haven't yet learned the Prius pattern, but it is more forgiving, has a different charging system and battery construction that charges slower, puts less demand on the battery, and has completely different failure patterns than we are familiar with in nonhybrids.

Much of what is known about weak 12V in Prius is recorded in Weird stuff happening? Test The Battery: Here's How.

 

High speed is a bit of a "leveler" for the Prius. It still gives good fuel economy, but the relative difference to other cars is not so pronounced as it is at lower speeds. Obviously given the length of your commute I'm not going to suggest slowing down. As an experiment however you might like to try a "one off" trip at 70 MPH and you will see an improvement. (BTW. Do this test on your way home some time when your mother-in-law is visiting. )

Your MPGs will definitely improve a bit with warmer weather and LRR tires. I think you will be able to top 40 fairly easily. One tip I'd give for those speeds in hilly terrain would be to NOT use cruise control over the very hilly portions of your route. You'll strain the engine a lot less and get better MPG if you drive the hills "manually" using the usual technique (for a slightly low powered car) of speeding up a bit just before the hills and allowing your speed to drop a little during the climb.

Testing the battery after it's been sitting for at least a few hours is the right way to do it. Those voltages are definitely a bit low, but the cold weather might be partially to blame.

 

You never answered the question about what tires you are running now. We are running the Michelin XIce-2 Winter Tires and exceeding 50 MPG with a Gen III Prius. That said, we are not running at the same speeds or in the same freezing temperatures that you are. As one of the original replies said, its the tires, the speed, and the cold.

 

Correct.

SOME failure modes cause reduced MPG. If one cell fails, the inverter can (and does) charge all day long and not bring the battery up to the 13.9 v charging voltage. That energy did not go to moving the car. Other failure modes do let the battery charge but with reduced capacity to do real work, they do not lower MPG, but the car starts having electrical 'glitches'.

As the 12 volt battery is needed to start the computers and to hold relays in, so the HV Battery is brought online to turn the engine, you do not get symptoms you may be used to in other cars. Besides the self diagnostics you have already found, you can try using the power windows before you go to READY in the morning. Slow is bad.

 

EPA hwy estimate for your 08 is 45mpg so you're not actually doing all that bad considering your current operating parameters. The good news is it will get better once temps rise, you're able to switch out tires, and you learn some hybrid driving techniques.

 

OP - Your mileage given the conditions are about what I would expect. I have an 09 and in winter on the highway at about 75 mph I figure that my mileage would be in the high 30's and I do not have snow tires.

At least comfort yourself with the fact that you like the car and are getting better mileage than you would with just about any other car - perhaps you could do better mileage-wise with one of the 3-cylinder Geo Metros from the early 90's, but then again you would be driving a Metro - that would make your commute a real treat.

 

OP - Your mileage given the conditions are about what I would expect. I have an 09 and in winter on the highway at about 75 mph I figure that my mileage would be in the high 30's and I do not have snow tires.

At least comfort yourself with the fact that you like the car and are getting better mileage than you would with just about any other car - perhaps you could do better mileage-wise with one of the 3-cylinder Geo Metros from the early 90's, but then again you would be driving a Metro - that would make your commute a real treat.