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.
https://substackcdn.com/image/fetch/...3052x1996.jpeg

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!

Gas burned, carbon emitted
 

Another view:
https://substackcdn.com/image/fetch/...3052x1996.jpeg

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!

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).

It's funny, but he and aerohead would agree in principle and then quibble over it.

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.

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."

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.

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.

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.

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.

Waste is subjective. Should I shame people who do 50 because they could spend half the energy doing 30?

80 is at the boundary of what I find acceptable personally. What if the person doing 80 is in an EV? They are more efficient than a dually F350 at any speed.

The point of traveling is to arrive at the destination. The less I've aged in that process, the better.

What if you have 6 neighbors that have been hit, 4 of them killed, as they either walked or rode their bike, and only one was in the wrong? Do I even dare walk or ride my bike? My job is almost exacly one mile away, 20 minutes walking and easily 10 minutes or less riding a bike. I'm also getting about 20mpg or less right now in the hybrid Avalon. But I don't want to die just because everyone else drives like an idiot.

I attempt to live statistically, not anecdotally, to the extent I can manage.

Amazing. I wondered why those crazies were driving 5 ft behind the forward cars bumber at 74 mph. !! This makes sense. Also, great points about the higher costs of speeding - very true.

Thank you for your civil engineering efforts to improve our built environment. You and your colleagues HAVE made a difference and we appreciate that. But yes the inevitable foolishness of a portion of our populace must dishearten you, all.

But I have noticed things like raised bump up red crosswalks make a difference.
Consider Netherlands (always consider Netherlands):

"In the Netherlands, the sustainable safety approach differs from Vision Zero in that it acknowledges that in the majority of accidents humans are to blame [emphasis mine], and that roads should be designed to be "self-explaining" thus reducing the likelihood of crashes. Self-explaining roads are easy to use and navigate, it being self-evident to road users where they should be and how they should behave. The Dutch also prevent dangerous differences in mass, speeds and/or directions from mixing. Roundabouts create crossings on an otherwise 50 km/h (31 mph) road that are slow enough, 30 km/h (19 mph), to permit pedestrians and cyclists to cross in safety. Mopeds, cyclists and pedestrians are kept away from cars on separate paths above 30 km/h (19 mph) in the built up area. Buses are also often given dedicated lanes, preventing their large mass from conflicting with low mass ordinary cars.

More recently the Dutch have introduced the idea that roads should also be "forgiving", i.e. designed to lessen the outcome of a traffic collision when the inevitable does occur, principles which are at the core of both the Dutch and Swedish policies.[33]

-- from Wikipedia entry on Vision Zero

PS I think this thread has morphed from aerodynamics to "general efficiency" and not sure how to deal with that.

Yikes. You are scaring me. I ride my Yamaha Cross Connect eBike EVERYWHERE and I am mixing with the "bigs" - cars, trucks, etc. I am lit up with every light and neon green overlays and ride safely but friends and recently my primary physician are shaking their head at me. Maybe I have just been lucky doing this for now !!! 52 years. Prob'ly so.

That is one thing that would probably help is just being seen. One of the neighbor ladies that got killed liked to wear all black. A guy was turning left and as soon as he had a green light he ran right over her in the crosswalk. She was in the adjacent crosswalk with the rightaway, but it also happened at night, so she was hard to see.

Thanks. Breathing easier.

While the doctors are fussing over my INR ratio, I tell them I'm in more danger from the trip to and from the clinic.

I have been wanting an Arcimoto FUV for it's cage, but there goes the exercise. Their micromobility solution, when it comes will be more lethal than a bicycle, zippy with hard cornering capability.

I’m
Surprised all that happens in gunnison. Stayed there in February to go ride at crested butte and it’s one of the smallest cities in Colorado I’ve ever stayed in. Barely even had an Arby’s to eat at or real grocery stores. I’m assuming it’s tourists who are doing the bad driving when in town.

45 is probably a little more efficient than 50, especially with low rolling resistance tires. I only did a steady state run for 20 miles in my Ioniq at 50mph with cc on and I was getting 87-94mpg on my trip reader. I was also ticking off 18 wheelers having to pass me in the right lane on the interstate on that test run. If I did 45 with cc on, my cars mpg reader would probably just say 99.9 the whole time and I’d never know how high it gets. Either way, going 50 and getting over 90mpg is fine. I’m not so cheap where I’d have to drop down to 45mpg to save even more money in my Ioniq…

At least four of the crashes were by local drivers. One guy actually ran over his own long time friend on a bicycle there in Crested Butte no that long ago. I knew the guy for a long time too as we had worked together at a few different jobs.

On the way to Bonneville Salt Flats, I ran through Eastern Oregon in the middle of the night, to maximize the overall mileage for the trip, in the Superbeetle -- 34MPG.

https://ecomodder.com/forum/member-f...3-100-0884.jpg

One stop for gas in 1500 miles, plus top-ups on the side of the road.

Instead of rebuilding the engine that needs a refresh, I shall use the Mileage Motor I've had on the bench fo a decade. That should be good for 40MPG before any aero mods.

1994 Nissan Sentra SE-R running 185/75/14 tires instead of the normal 185/60/14 tires.

I´ve been averaging about 41 mpg for the past two years with most of my driving between 55-60 mph. But, I always seem to run the engine to 6500+ rpms a few times on each tank. I took two 2500 mile trips over the past 3 years where I spent most of the time at 80 mph on the I-40. I got 35 and 36 mpg on all those tanks of gas. And I used the air conditioning between Amarillo and Memphis.

One of these days I hope to do some aerodynamic work on the car.

I had to go look up aspect ratio to understand your tire sizing difference above. So you are driving a taller tire. I had not realized that one could do that and get better MPG.
Here's what ChatGPT told me about it, "A taller tire with a higher aspect ratio (the ratio of the tire's sidewall height to its width) can potentially lead to better gas mileage because the taller tire has a smaller contact patch with the road, which can result in less rolling resistance. However, it's important to note that a taller tire also has a taller gear ratio, which can result in a lower top speed and can negatively impact acceleration. Additionally, taller tires can also negatively impact the vehicle's handling and stability. So, it is important to consider all factors before making a decision to change the aspect ratio of the tire"

I will take the better MPG.
Thanks,

rolling resistance
 

With some tires, up to standing-wave velocity, the power absorption coefficient is constant, providing a straight line power 'curve.' ( 1996 Honda Accord )
Other tires' coefficients vary with velocity, producing a 'curved' horsepower curve with speed.
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PS: The standing wave occurs above the tire's speed rating, which is above the car's top speed, so technically, one could never experience the self-destruction which can accompany standing wave.

First use I've seen of that particular argument from authority

Neither arguing nor appealing. Just interested to share how a higher aspect ration can reduce the amount of rubber touching road.
Did not mean to affirm or deny authority here.
Just trying to add.
JD

'from authority'
 

It's caused a lot of trouble here at EcoModder.

No judgment. I was just pointing out the use of ChatGTP as a resource. Implying it fallacious was my bad.

I posted to this thread because I feel I´ve done some decent testing about the difference in mileage my car gets at 55 mph versus 80 mph.

Yes. When I bought the car I was getting 32-35 mpg. As with all the other 4 cylinders with 5 speeds I´ve had I wished it had a 6th gear for better mileage on the highway. With the normal size tires the engine runs at 3000 rpm at 60 mph. In a less than 2500 lb. car with 140 hp and 132 lb-ft of torque I think it´s ridiculous to run the engine at such high speeds on the highway. I wish it turned 3000 rpms at 75 mph. That would rarely cause any problem. I´d probably have to use 4th gear in some places like west of Denver on I-70 but most of the time it would cruise just fine in 5th gear on four lane highways.

I´ve checked the odometer/speedometer error at least 10 times on the Interstate over 100 miles. With the taller tires for instance, I would pass mile marker 100 with my trip odometer at 100.0 miles. When I reached mile marker 200 my trip odometer would read 192.4 miles. I´ll check it occasionally over 1 mile and I see the trip odometer read a little over 0.9 miles and I do ten mile checks sometimes and see that the odometer reads less than 9.3 miles. So for speed corrections I divide by 0.93 to know my actual speed although I´m erring against what might be the true mileage and speed. 60 mph / .93 equals 64.5 mph. At 93 mph on the speedometer I should be going a little over 100 mph.

The contact patch should be the same since both tires are supposed to be 185 mm wide?, but the 75% ratio tire does appear to have a little narrower contact patch. You get slight variations in tire dimensions even though they are labeled the same.

I doubt there is much difference in top speed with this car. The coefficient of drag on this car isn´t that good. Total drag probably has the most influence on that. I can tell the acceleration is negatively effected. The effect is little enough I have no concern about it. With the taller tire the car is a little higher off the ground so I´m probably increasing the drag a little bit.

The taller tire does have a bad effect on the cornering. That´s not enough to bother me. It still drives good. The taller tire also had an effect on the ride quality. It rides smoother and quieter. I forgot to mention I´m using the alloy wheels off a 1999 Sentra. They are lighter than the original wheels so that should help the handling a little and have a good effect on the ride quality. I have no idea whether the taller sidewall or the lower unsprung weight improved the ride quality most. I doubt the weight difference has much effect on the mileage. I´ll measure the difference in weight some day. I also had to use spacers on the rear wheels to push them out from under to spring perch otherwise the tires would touch the underside on the perch. That´s likely to wear out my rear wheel bearings sooner. I´ve had no problem with clearance on the front wheels but I have had the rear tires rub two or three times but I was carrying several hundred pounds if not as much as 1000 lbs. in the car.

When I first changed the tires, I would get the same mileage calculations from how many gallons it would take to fill it compared to the reading on the trip odometer. 1 / 0.93 = 1.075. I´m now traveling 7.5% farther than the trip odometer measures and going 7.5 % faster than the speedometer measures. 35 mpg / .93 = 37.6 mpg. I´ve learned to drive the roads in my area better so I´ve gotten better mileage than when I first moved here. I´ve also replaced several engine parts trying to get it ruuning as good as possible. I do coast some times in neutral with the engine running and I keep my eye on the traffic lights trying to stay in 5th gear as much as possible. It´s kind of amazing how little brake pads I use compared to 30-35 years ago.

I´m not as exacting as many people on this forum but I think my measurements are decent. I´ve not recorded less than 41 mpg the last two years except for the two high speed trips on I-40. I´ve calculated 42 mpg two or three times, 44 mpg once, and a remarkable 47 mpg once in the past two years. I always try to use 10 gallons before I refill the tank to reduce any errors from the difference in the level I fill the tank. I´d doubt difference in the level I fill the tank is ever more than 0.2 gallons. With my best mileage, the pump indicated 10.25 gallons. I calculated 492 miles / 10.3 gallons for 47.76 mpg. If I did a poor job filling that tank and actually used 10.5 gallons that would only change the mileage to 46.86 mpg.

That´s something I might argue is wrong. The first number 185 is supposed to be width of the tire in millimeters. The second number is the percentage of the width that the tire should measure from the tread to the bead? I have no idea where the width measurement is supposed to be taken, the tread or near the widest points on the sidewall?

Wonder what a better tire size for my Ioniq would be. I have the eco 185/65/15s. Wonder how a skinnier tire at like 155 or 165 would affect mpg.

You'll run out of tires before brakes, downhill in the mountains.

How does the vehicle weight compare with a BMW i3?

No idea. But the hybrid is 3000 pounds. The guy at Costco said that’s why my tire treads never last as long as the advertised miles for tires. Said the hybrids are too heavy

It looks like you could go to tall narrow 19" tires.

The more premium ioniqs come with 17 inch tires. The eco one I got has 15 inches. Obviously that means I should have a little wiggle room to fill in the wheel arch gaps

155 / 165
 

*GM used tires as narrow as 145mm with their Aero 2002/ 2003A concepts.
* The tire circumference to wheel arch gap needs to remain constant.
* The outside face position of the sidewall needs to remain constant.
* The load rating needs to be equal.
* Lateral G-Force loading / adhesion ( skid-pad ) capability needs to be equivalent.
* Speed rating needs to be equivalent.
* Dry traction rating ( compounding, tread design ).
* Wet traction rating ( compounding, tread design ).
* Temperature rating.
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Wheel fairings can reduce aerodynamic wheel drag by up to 70%.

It's going to be hard to find an eco tire that's skinnier than 185 without sacrificing diameter. A 185-width low rolling resistance tire will be more efficient than a 155-width tire with high rolling resistance.

You must not have put Permalink #33 and #34 together.

ecomodder.com/forum/showthread.php/bridgestone-announces-large-diameter-narrow-tires

Anecdotally:

-In my stock Insight, fuel economy effectively stopped improving below 50mph - ~100mpg was my effective cap regardless of whether I was driving at 30 or 50. It *appeared* that as speeds dropped I was more affected by things like climbing hills or needing to brake for turns, proportionately. Presumably, parasitic losses also became a larger portion of total energy used, relative to distance traveled.

-In New Zealand, the national speed limit is 100kph, and there is virtually no culture of speeding. In fact, people often drive 10-20 under the limit, and there is a reasonably strong push to drop the national limit down to 90 or 80.

I use over inflated ecopias in the summer. Right now I got my sticky gummy bear winter blizzak tires on right now. Probably can’t get anymore high rolling resistance unless I had studs or added chains lol