Question: RE Prius mpg
 

Some of you may recall the bet I had with the co-worker on the truck mpg... chatting with him day to day during the bet got his attention on mpg improvements and he since started watching his Prius mpg on the dash monitor. Today he tells me he stumbled onto something that has both of us scratching the head...

Ambient temps in the mid 40's, his monitor tells him he's getting 42~43 mpg on the way to work. Consistently, temps around here are pretty consistent with the variation only being 5~8* during the day, most days.

So the other day temps dipped a tad bit more and he had frost on his windshield. So he cranked on the defroster on high temp. By the time he got to work, he was in the 47~48 mpg range. Catching his attention he then started cranking on the heater on high before he leaves home to see how repeatable it is. And it appears to be repeatable according to him. I told him to fill up the tank, then do an A-B-A test with his findings to see it it tracks what he saw.

Anyone heard of this?

Is he running on batteries longer which is driving up the mpg #'s?


I joked with him saying "so I go to all these lengths to drive up my intake temps and do additional various things to drive up mpg and all you have to do is turn on the heater..." his neighbors started lol...

Sounds like placebo to me. There is no reason that using more energy (by using the heater) would improve mileage. It should actually lower it since his engine is now going to take longer to warm up. My only thought is he is more comfortable, and thus relaxed driving with being warmer. This may lead to better driving habits.

He says he's running on batteries at that time & the only time the motor kicks on is when he's going uphill. I have zero knowledge of how the hybrids work.

My thought was the heater being on high is causing a high current draw which will increase the battery temps. How that = better mpg in the mornings I have no idea...

Wind conditions?
Wind hurts my economy worse than cold does. Then, a cold spell often happens during a lull in the wind, like mild cold dry eastern instead of hard wet south-western wind.

... and that's how superstitions are born. Don't control for any variables, decide to attribute outcome A with action B, and you have yourself a ritual.

His lie-o-meter is telling him 51 mpg today, ambient temps up slightly ~2-3*f vs earlier in the week.

He plans to do a fill & drive for a week under both conditions.

ON another note, I've been told records tell the tale, I've kept my fuel receipts over the years and have seen my mpg go from a peak of 19.5~20 mpg in summer up to 22.5~23.3x mpg. From what I've seen, it doesn't take but a few fill ups to see if a change has affected mpg. My current goal was to keep mpg up above 21 mpg under ideal winter conditions. Achieved... 21.6 mpg on a short fillup with all hiway miles. Added a CAC blanket to drive intake temps up as high as I can get them. What was 70's all day everyday, is now peaking up into the mid 90's and running at mid 80's once up to temp.

Can't be. On my commute in my Prius when temps are around 30-40 in the morning on my 10 minute commute if i don't use the HVAC the engine will stay off longer and I will achieve 40-51~. If I turn on the HVAC pretty much kiss that goodbye. I typically get something like ~29 mpg with the heat on. For that reason, I just wear a nice coat.

Can be... there is nothing flat on his commute to work... mine has just 1 or 2, 1~2 mile stretches on the freeway and some surface streets... I have no reason to doubt his lie-o-meter claim. Whether his claim is accurate or not will depend on what his fill up calculates out to.

That's still empirically pretty meaningless; he'll only be controlling two variables (route, defrost on/off) out of many (ambient temperature, windspeed, wind direction, co-directional traffic, anti-directional traffic, stop and idle time).

Actually, just one if he does any driving at all outside of his commute.

The meaning of my first post is that conducting a non-controlled, not scientific test, and trying to derive scientifically valid conclusions is impossible.

If your friend truly cared about the hypothesis "cranked on the defroster on high temp...results in improved MPG", he would run the experiment and control for as many variables as possible. You don't need full tanks to run the test, just a reasonably accurate gauge to measure fuel consumed, or trip MPG.

well in a Prius C if cut defroster on high, engine stays running, if lower setting or just heat, I see engine cycling normally until warmed up, but not done any official testing, just my own observations on my wifes car, which I don't drive much, which would be a hair worse mileage when on high defrost, while the engine is warming up.

That might be it right there... I have no idea how his car works or how accurate the lie-o-meter is. He did mention when he first leaves normally, the motor is off. I think I'll go grab him when he gets in and go investigate :D

Your right, his commute is a mere 13 miles but can take him over an hour going home.

This is his case... motor stays running with defroster on hi, not sure about other settings.