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Discussion Starter · #1 ·
A rare partial exception to the general rule of "slower = better mpg":

Since getting our Prius in February 2005, my wife have driven to the same yoga studio 60 or 70 times. It's about 10 miles away and uphill from our home. We always take the freeway to class (because we’re usually running late!), but we have a choice of routes home:

• Route A is back down the freeway;
• Route B meanders through residential areas, with more stops and lower speed limits.

Of course, we get better mpg in warm weather than in cold weather — but the exact effect differs for the two routes. In warm weather, we get our best mpg on Route B. Even though there are numerous stops, it doesn’t take much energy to get the car up to 30 mph, and much of the time it just glides. I once gassed the car up after class; the MFD read 97.1 mpg when we got home. In the same conditions we get about 85 mpg on Route A. (It IS downhill.)

But when the temperature is under 30 degrees, we actually get slightly better mpg on Route A. On Route B, the engine has hardly any work to do, but it keeps puttering anyway in order to warm the catalytic converter, and it putters for a long time. On Route A, the engine is working harder, but more of that heat is being used to move the car. So in cold weather, we get a little over 70 mpg on Route A, and a little less on Route B.
 

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For the 10 mile commute, that all makes sense to me. If it were 20 miles or more, route B might do equal or better in cold weather since the ICE/catalytic warms up.

And as I find, I'm sure going up that hill uses twice the gas as what you recover coming down...for example, if the MPG reads 50 at the start of ascent and 48 at the top, if you go right back down, you should end up at 49.
 

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Discussion Starter · #3 ·
wayneswhirld said:
For the 10 mile commute, that all makes sense to me. If it were 20 miles or more, route B might do equal or better in cold weather since the ICE/catalytic warms up.

And as I find, I'm sure going up that hill uses twice the gas as what you recover coming down...for example, if the MPG reads 50 at the start of ascent and 48 at the top, if you go right back down, you should end up at 49.
It seems to depend on the hill.

• Our Prius got great mileage on a trip on I-70 through the Eisenhower Tunnel: With four people and temperatures mostly in the 30s and 40s, we started the day at 53.something mpg and gassed up 170 miles later at 54.8, despite a net climb of a few hundred feet. That means we got in the upper 50s for that stretch, comparable to our usual highway mileage.

• But it didn't get great mileage in Rocky Mountain National Park, where the long, slow descent maxed out the battery miles before the bottom of the hill. We lost a lot of kinetic energy using "B" mode while showing eight bars.

Of course, it's hard for me to judge since I live on the front range of Colorado, where there are basically no level roads for comparison! As for our yoga studio mpg, it's usually higher when we get home than when we left - but it's one of the longer trips we make in town, and as you know, trip length trumps just about everything else when it comes to mpg.
 
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