Bad mileage myth?

[Marlin]

Originally posted by DocVijay@Mar 14 2006, 12:29 PM
But I also make short trips all the time. The gorcery store, my kids daycare, and many other things are all within 5 miles of my house. I make plenty of short trips with a cold engine. But I still get good mileage.

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You might make plenty of short trips, but a far greater percentage of your tank is very long trips.

As near as I can tell, the Prius must spew uncombusted gasoline out the tail pipe when it's warming up, because the milage during that time is really, really bad. For me, it's typically 35 MPG in the summer for the first 5 minutes and 25 MPG in the winter for the first 5 minutes.

So, I think it's well established that during warmup, the engine's efficiency really sucks. Now let's examine that "I make plenty of short trips with a cold engine" comment.

My wife and I each have a Prius. I've been getting 48 MPG or so for the last couple of weeks and my wife is getting 35 MPG. When it was colder, I was getting 43 and she was getting 30.

Now lets compare her drive and my drive. I commute 16 miles for about a 30 minute commute. On the weekend, I might make 3 or 4 short trips of less than 10 minutes. So, monday through friday, I spend 5 minutes getting bad gas milage and 25 minutes of really good gas milage for each leg of my commute. So, during the week, I spend 50 minutes getting bad gas milage and 250 minutes getting really good gas milage. On the weekend, my 4 or so trips of less than 10 minutes contributes an additional 20 minutes of poor gas milage and 20 minutes of good gas milage, resulting in a one week total of 70 minutes of really sucky gas milage and 270 minutes of really good gas milage.

Therefore, for my driving, I'm getting good gas milage for approximately 80% of my driving.

Now lets look at my wife's driving. She is a stay-at-home mom and primarily goes to gym, drops my daughter off at kindergarten, and goes to local stores. Most of her trips are 7 minutes or less, and many are less than 5. I'm not going to try to do the math, but it should be obvious that for her driving patterns, instead of driving 80% of the time with a warmed up engine getting good gas milage, she is probably driving closer to 80% of the time on a cold engine getting bad gas milage.

Now you, with your 120 mile commute, even with local trips in the evening or on the weekend, are probably driving approximately 90% of the time with a warmed up engine getting good gas milage. That's why you get good gas milage without even trying and why other people can't get good gas milage no matter what they do.

I would bet that if everyone accurately estimated the percentage of time they drive with a cold engine and the percentage of the time they drive with a warm engine and then used the following formula, they would come close to their real world milage.

Pcold * 60 + Pwarm * 35 = mpg (for summer temps)

Pcold * 50 + Pwarm * 25 = mpg (for winter temps)

Where Pcold and Pwarm are between 0.0 and 1.0. (80% = 0.8)

[DocVijay]

Well, yes, the flatness does also play a part.

Now, as Dave pointed out, the A/C use also has a big impact. In Florida we use A/C every day. The recent figures I've pointed out are WITH the air conditioner. Even in the lower 80's, a car gets very hot here in FLorida after sitting in the sun for hours and hours. I always use A/C.

The other point that I forgot to reinforce, is the larger, heavier wheels. When I proposed it (and others have too), there were numerous replies about the steep drop in mileage I would get. The Scion tC wheels/tires each weigh just over 10 pounds more than the stock Prius wheels. Some people posted apocalyptic results on my mileage. Yet those have not materialzed. Overall I only lost about 0.7 mpg so far. The wheels tires must be factored in as well.

So, with ALL these factors taken together:

- significantly heavier wheels/tires, larger tire contact patch, higher rolling resistance
- constant A/C use (set to 72 degrees)
- long drive with lots of stop/go congestion
- inefficient driving
- flat terrain
- mid 80's temperature

Consider all those and then look at my 51 mpg. Consider all those and that I TRIED to get the lowest possible and got only 40 mpg, and it's even more impressive. You really have to do a lot to get those really low numbers. Now if it's well below freezing and you only make a short (<5 miles) trip, then yes, I guess getting 35-40 is reasonable. But there are some who are getting mid 30's and it's considerably warmer. There are some who are complaining of getting much worse mileage on longer drives. Those are the ones I'm questioning.

[EricGo]

Outside temps matter (~5 mpg at in low 20's F, vs > 50F), but engine temp matters a whole lot more. I drive many jaunts of 10 miles in 25 - 35F weather through the winter, usually with 2 - 5 hour intervals when the car gets to cool off. IF I wait to turn on heating for 5 minutes, and only use it on uphills or accelerating, my FE is about 55 - 60 mpg at night with no traffic to speak of in suburban/city driving. For comparison, if I use AC with abandon, mpg is in the 40's (I know from my wife's habits), vs 65 - 70 mpg in ideal weather.

[andreaswin]

well 40 pound ca18kg right ?) in total isnt that bad is it ?
I Guess there is many factors that will affect how much more gas your car will use when putting on heavier weels,, and that would affect more in acceleration gas usage and performance , when ure in desired speed it shouldnt affect the gas/milage that much.
And i theory you sould get alittle more back in power to the batteries when ure deacceleration ( "flywheel " effect should affect both ways.....)
It also depends where in the weel You have the new 10 pound weight , distributed allover in same pattern as the original weels or ?? and so on..
my experince is that the tire itself plays a big role in gas milage. the pattern of the tire, profile bla bla

i might be utterly wrong ,
aslong it looks good on the car and not made of lead ..... im happy

*Andreas

Originally posted by DocVijay@Mar 14 2006, 10:22 PM
Well, yes, the flatness does also play a part.

Now, as Dave pointed out, the A/C use also has a big impact. In Florida we use A/C every day. The recent figures I've pointed out are WITH the air conditioner. Even in the lower 80's, a car gets very hot here in FLorida after sitting in the sun for hours and hours. I always use A/C.

The other point that I forgot to reinforce, is the larger, heavier wheels. When I proposed it (and others have too), there were numerous replies about the steep drop in mileage I would get. The Scion tC wheels/tires each weigh just over 10 pounds more than the stock Prius wheels. Some people posted apocalyptic results on my mileage. Yet those have not materialzed. Overall I only lost about 0.7 mpg so far. The wheels tires must be factored in as well.

So, with ALL these factors taken together:

- significantly heavier wheels/tires, larger tire contact patch, higher rolling resistance
- constant A/C use (set to 72 degrees)
- long drive with lots of stop/go congestion
- inefficient driving
- flat terrain
- mid 80's temperature

Consider all those and then look at my 51 mpg. Consider all those and that I TRIED to get the lowest possible and got only 40 mpg, and it's even more impressive. You really have to do a lot to get those really low numbers. Now if it's well below freezing and you only make a short (<5 miles) trip, then yes, I guess getting 35-40 is reasonable. But there are some who are getting mid 30's and it's considerably warmer. There are some who are complaining of getting much worse mileage on longer drives. Those are the ones I'm questioning.

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[DocVijay]

Originally posted by andreaswin@Mar 14 2006, 07:53 PM
well 40 pound ca18kg  right ?) in total isnt that bad is it ?
I Guess there is many factors that will affect how much more gas your car will use when putting on heavier weels,,  and that would affect more in acceleration gas usage and performance , when ure in desired speed it shouldnt affect the gas/milage that much.
And i theory you sould get alittle more  back in power to the batteries when ure deacceleration ( "flywheel " effect should affect both ways.....)
It also  depends where in the weel You have the new  10 pound weight , distributed allover in same pattern as the original weels or ?? and so on..
my experince is that the tire itself  plays a big role in gas  milage. the pattern of the tire, profile bla bla

i might be utterly wrong , 
aslong it looks good on the car and not made of lead ..... im happy

*Andreas

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Yes, 40 pounds is not a lot of weight, but this is a rotating mass, and is unsprung weight. These two things have a much greater affect on mileage than putting 40 pounds in the trunk, etc...

You are also right in that in highway driving the mass has a lesser effect, but you really never go at a "constant" speed. It always varies some. But it is less than in city driving. I do both though, so any benefot on the highway is offset by city driving. Quantifying the exact tradeoff balance is pretty impossible to quantify without some seriously advanced telemetry...

I also considered the "flywheel" effect as you stated. Once again I really don't have any way of quantifying it though, so your guess is as good as mine. Well, unless your're an engineer, in which case it's probably a little better than mine.

Also, the distribution of the mass is definitely a factor in the amount of energy required to change the rotational speed. If the weight is concentrated farther out, as it is in a plus size wheel/tire, it is harder to change the speed, i.e. more energy to accelerate. If the mass is conentrated more towards the middle of hte wheel, it takes less energy to change the rotational speed.

Rotational friction from the tire is actually static friction, as the surface of hte tire is not sliding along the pavement (at least not ideally...). By increasing the tire's contact patch you do increase this friction.

However, the greatest source of frictiona nd power loss for a car is thorugh air resistance. A lower Cd certainly helps, but at a certain speed more energy is spent fighting this frinction from the air than any other source. Speed definitely kills (your mileage).

So to sum up, I agree, if it looks good and isn't made of lead, it's all good.

[DocVijay]

Originally posted by EricGo@Mar 14 2006, 04:08 PM
Outside temps matter (~5 mpg at in low 20's F, vs > 50F), but engine temp matters a whole lot more. I drive many jaunts of 10 miles in 25 - 35F weather through the winter, usually with 2 - 5 hour intervals when the car gets to cool off. IF I wait to turn on heating for 5 minutes, and only use it on uphills or accelerating, my FE is about 55 - 60 mpg at night with no traffic to speak of in suburban/city driving. For comparison, if I use AC with abandon, mpg is in the 40's (I know from my wife's habits), vs 65 - 70 mpg in ideal weather.

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Well as I stated earlier, in cold climates, a block heater can do a lot to help those winter mpg blues.

As for the A/C, when it's 95 degrees and 100% humidity, you have no choice but to use it in Florida. Any mileage I quote is assumed to have the A/C on by default.

[DaveinOlyWA]

all in all, i have to agree with the Doc. the whole basis of this thread is not understanding how people can get very low mileage. sure there are extremes. the first winter i had my car, i was in a training class that was 1.1 miles away from my front door. it lasted 6 weeks. we had 60-90 minutes for lunch, most of which i spent at home, on the computer, here at Priuschat (bet that was a shocker to you!!) so that was nothing BUT short trips. and my mileage was in the mid 30's. but a few trips of 70-80 miles on the weekend was enough to bring me back to the low 40's.

back when i only had 3000 miles on my car, i helped a friend deliver the morning paper. that was over 160 stop and go deliveries. only did 4 days worth but during that time (i did fill up the night before i started the 4 day run) i averaged 41 mpg. granted it was summer time, but this is WA State and at 3:30 in the morning, it aint that warm no matter what time of year it is.

but there have been a lot of people who have 20-30 mile commutes on the freeway or some variation of, claim to drive very conservatively and still get mileage in the 30's. i have to side with DocViJay. i simply dont see how it could be done without driving like a maniac.

[daniel]

Originally posted by tripp@Mar 13 2006, 03:05 PM
It's baffling ain't it? I'd really like to go on a ride-along with someones who's in the low thirties or high twenty's. It would be really interesting to see what factors could lead to such low numbers.

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Recipe for 29 mpg:

Park your Prius in an unheated garage when the outside temperature is 30 degrees below zero Farenheit. Start the engine and turn the heat on full to pre-heat the cab for 5 minutes before driving, because at 30 below zero your fingers would freeze and break off. Drive 4 miles in 10 minutes without the car ever getting into Stage 4 operation, and with the heater on full blast all the way, so the engine never shuts off. Park the car. Jog at the YMCA. Start up the car and drive 2 or 3 minutes to the office, and park again. In the afternoon, when the temperature has warmed up to a tolerable ten degrees below zero, drive back home, again with the heater on full blast, though this time without a pre-heat. Don't use the EV switch because you didn't have one that first winter when the car was new. But do accelerate hard because you are in a hurry to get back into a heated building.

And, no, you would not like riding along. It's too confounded cold. Which is why I don't live there any more. 29 mph is not too hard to achieve. Here in Spokane I'm getting 50 mpg and satisfied with that. Fargo's a nice place, but you have to be half polar bear to survive the climate.

[tripp]

OK Daniel. Good point. I've heard you and other like Jayman mention those conditions. That makes sense, no arguments here. However, it seems that people in much milder climes have experienced similar results. That's who I'd like to ride along with (no offense )

[daniel]

Originally posted by tripp@Mar 15 2006, 08:19 PM
OK Daniel. Good point. I've heard you and other like Jayman mention those conditions. That makes sense, no arguments here. However, it seems that people in much milder climes have experienced similar results. That's who I'd like to ride along with (no offense  )

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No offense taken. But has anyone actually gone below 30 mpg without bitterly-cold weather, or a one-mile commute? Take a look at your first 5-minute segment, and consider the person whose daily drive is 5 minutes.