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Old 01-01-2023, 02:31 PM   #1 (permalink)
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The real costs of 80 vs 55 mph

I used krousdb and MetroMPG's marvelous Aerodynamic & rolling resistance calculator - EcoModder.com to compare my 2009 Prius at 55 and 80 and see what the real costs were. I visualized its results

I was comparing my daughter's 80 MPH with my own 55 MPH. This chart shows that both my daughter, at 80 mph, and I, at 55 mph, spent the same energy, 115 watts, to overcome rolling resistance. But at 80 mph Margaret (my daughter) needed to spend over double (452 watts) the force I used (214 watts) to overcome the increased wind resistance. This brought her miles per gallon down to 28 mpg, 42 % below mine at 48 mpg.


Margaret, at 80 mph, would cover 150 miles in about an hour and 50 minutes while it would take me, at 55 mph, 2 hours and 40 minutes. However, to save those 50 minutes Margaret, speeding at 80 mph, burns 2.28 more gallons and adds 44 more pounds of carbon to the atmosphere than I would have at 55 mph.

This is no new news to anyone in this Forum, but it might allow me to teach My daughter to slow down!

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Old 01-01-2023, 02:32 PM   #2 (permalink)
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Gas burned, carbon emitted

Another view:
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Old 01-01-2023, 02:56 PM   #3 (permalink)
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That chart says rolling resistance doesn’t change with speed, but in another thread peiple were saying it goes up with speed but just at a much smaller degree

Another confusing thing is that Julian Edgar said that wind resistance doesn’t actually increase until you hit about 37mph

But I agree, slower on shorter trips is much better and losing a few minutes. But if you’re driving 900 miles in a day, the difference in time between 55 and 80 is huge!
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Old 01-01-2023, 04:48 PM   #4 (permalink)
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Rolling resistance would be dominated by bearing and the contact patches.

Those likely vary by temperature, both ambient and internal (hot bearings and tires on a cold day vs cold on a hot day).

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Another confusing thing is that Julian Edgar said that wind resistance doesn’t actually increase until you hit about 37mph
It's funny, but he and aerohead would agree in principle and then quibble over it.
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Old 01-01-2023, 07:44 PM   #5 (permalink)
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Quote:
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That chart says rolling resistance doesn’t change with speed, but in another thread peiple were saying it goes up with speed but just at a much smaller degree,,,,, wind resistance doesn’t actually increase until you hit about 37mph!
Good additions. I just took the model's output at face value and graphed it. Twould make sense that Rolling Resistance increases a little as more friction would indicate. Also interesting on the no wind resistance until 37 mph. I have often heard that 45 or so is the most efficient speed and the 37 would support that. But I know from riding my bike that wind resistance DOES increase from about 5 mph and up. By 20 MPH I can really feel it. (Empirical evidence).
Thanks for the corrections.
JDaly

PS. I am getting grief from friends who drive a lot who say they'd go crazy driving any slower than 65 MPh. Think driving from Boston to Buffalo like that.
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Old 01-01-2023, 08:19 PM   #6 (permalink)
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PS. I am getting grief from friends who drive a lot who say they'd go crazy driving any slower than 65 MPh. Think driving from Boston to Buffalo like that.
If only you could take everyone's car away for a month and then tell then that if they want to have a car again it has to be governed to go no faster than 60mph and see what they'd say then.

I also knew a guy that when one time he was running out of fuel he started driving as fast as he could "so he wouldn't run out of gas before reaching the gas station."
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Old 01-01-2023, 08:39 PM   #7 (permalink)
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If only you could take everyone's car away for a month and then tell then that if they want to have a car again it has to be governed to go no faster than 60mph and see what they'd say then.
Yes. You are right. People are accustomed to fast driving (~70 mph plus) and the peer pressure on the roads makes it seem normal. But its wasteful. More than a few communities (Bristol England, Montgomery County MD, others) are endorsing a 20 is plenty campaign for congested suburban / urban areas..."The 20 is Plenty campaign is a part of MCDOT’s Vision Zero efforts to make the County safer for pedestrians, bicyclists, and motorists. The campaign sets the speed limit to 20 mph in designated areas as lower speeds lead to less-severe crashes. "

Hard to change behaviours and expectations though. Like smoking.
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Old 01-01-2023, 09:00 PM   #8 (permalink)
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I wish more would be done in my town/country/world to favorbetter alternatives. The streets in my town are a pedestrian's/biker's nightmare. Reduce all the wide lanes on these streets to tight lanes that weave and have speed bumps and make nice big, protected biking and walking lanes. Actual sidewalks would be nice. The streets are so wide here you could do all that and add an additional "emergency vehicle only lane" right down the middle too and still have parking on both sides of the street.

Make faster, cheaper, safer alternatives. Put in a bullet train to get to the big city. Try seeing if your daughter will want to race a 275mph bullet train that uses way less fuel/electricity per passenger than everyone in cars.
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Old 01-01-2023, 09:14 PM   #9 (permalink)
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I wish more would be done in my town/country/world to favorbetter alternatives. The streets in my town are a pedestrian's/biker's nightmare. Reduce all the wide lanes on these streets to tight lanes that weave and have speed bumps and make nice big, protected biking and walking lanes.
You and I are in strong agreement. Now to convince another 250 million folks and begun redesigning - urban planning? civil engineering?
We've got a lot of convincing to do.
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Old 01-01-2023, 09:49 PM   #10 (permalink)
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If you really want to make this a fair comparison, include all added yearly driving costs and assume she gets one speeding ticket a year or every other year.

Then jack up her insurance rates from the 1 ticket per year and add the costs of the speeding ticket to that. This could easily be another $2000 per year. She should also use a shorter oil change interval, oil brakes down faster at higher rpms, and her brake pads and calipers and wheel bearings will wear out faster, so add those shorter maintenance intervals in.

Now add in a minor wreck every three years (she will be okay, its a safe car) from following people too close trying to keep her intended speed. People who speed tailgate other drivers doing the speed they want to drive so slower drivers do not pull out in front of them.

I have been doing road design and Civil Engineering for years. You cannot Engineer out the stupidity of drivers or pedestrians or slow people who want to speed. About all you can do is remove obstacles out of the shoulders and clear zones so when they drive off the road while looking at their phone, they don't hit something and sue you. You can design and build the safest intersection in the world with flashing lights and narrowed road, safety bulb outs, etc. and some idiot who is high will cross the street in front of the car at night dressed in black and get hit and still sue the driver and possibly the Engineer.

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