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Thursday, 20 December 2018


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It is surprising what difference to MPG/KmL speed can make, if I do 50-60 on a suitable journey I can get 70+ MPG (UK gallon). Hammering it at 75+ (130KmH French autoroute) it goes drastically down. There is a community of people; HyperMilers but a fair bit of what they suggest can be a bit dodgy.

The price for speed, which I will pay, as I do find my attention wanes at 55 MPH.

I'm not sure I fully agree with your calculations, Mike. I suspect the online calculator is making certain assumptions. My Subaru Forester often tells me I'm getting 30mpg at 80mph on an Interstate highway and mid-20's at 55mph. But that's anecdotal. Let us know in a few months if you actually enjoy the savings you expect. Just promise me that you'll keep to the right whenever you're on a four-lane highway. Don't become that driver.

Mike, you've lost me here. If 10% gas savings is $1,200, then you spend $12,000 a year on gas! If gas is $4/gallon, you use 3,000 gallons per year, or about 58 gallons per week. You must either be a driving fool or be into the new Fuzzy Math!

Say it ain't so, Mike!


You'll save time not having to refill your fuel tank so often!

Here's another calculation: If you drive 50K miles per year, as I used to, the difference between 55mph and 70mph is 8.125 24hr days behind the wheel. 8.125 fewer days to spend with family, read, take pictures or what-have-you. Economy be damed is what I used to say. Time trumped money, at least when I was traveling extensively. But bravo to you for making us aware of the difference.

Either your math is off, you drive a gas guzzler, or you drive over 100,000 a year. If your efficiency improves 10% by driving 55 instead of 65 and you save $1200, you are otherwise spending $12,000 a year on gas, which at $2.65 a gallon and 25 miles per gallon means you drive 113,207 miles per year. Or thereabouts.

You might want to double check that math ...

If you drive a whopping 20,000 highway miles and get 30 mpg, you're using 667 gallons. Drop that by 10% (27 mpg) and you're at 740 gallons. At $3/gallon, that's an increase of $220.

Your 2.5 minutes estimate suggests a 15 mile round trip, which only adds up to 5000 or so miles a year.

One of the things I looked in before picking our last car is diminishing returns with fuel consumption. You save about as much going from 15mpg to 18mpg in an SUV as you do going from 30mpg to 45mpg in a car. It's kind of like megapixels ... those big numbers sound impressive, but they don't really buy you that much.

I was getting excited about saving $1200 a year till I read that the lower limit was 55mph. Highest speed limit here is 45mph.

But to share a tip with you - we bought deer whistles for our vehicles at Amazon - https://www.amazon.com/gp/product/B003XNGLJU/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1. We have the highest deer per acre ratio in Massachusetts. Haven't seen a deer on our way to town since installing them.

I'm sorry, Mike, but just off the top of my head, something seems off with the numbers you've cited.

Assuming gas costs $4.00 per gallon in your area, if improving your car's gas mileage by 10% saves you $1,200 per year -- i.e., 300 gallons of gas -- then you must have been burning 3,000 gallons of gas previously.

With your car being rated at 31 mpg highway, doing a bit of number crunching comes up with you covering an annual mileage of more than 83,500 miles!

While I suppose that's possible -- are you now driving for Uber in addition to blogging? -- it seems unlikely you're averaging ~1,600 miles a week at 16 miles per round trip. (scratches head)

I can't drive 55 where I live, as I will be killed by either falling asleep or being rundown by other cars. But for you in the densely populated NE US, understand that things are MUCH farther apart than where you live, and, as Mike might agree, time is money.
I have owned two cars for which I closely calculated gas mileage, one a Mercury Capri, the German made version, and the other, that I currently own, a Lexus Coupe. In both cases those cars get or got, measurably better mileage at 70 than at 55. At least a additional 5mpg in both cases. In the case of the Capri, it got it's best mileage at Autobahn speeds. So when they say YMMV, they really mean it. And who are they?

Being a fan and listening to “Car Talk” (NPR) for many years, it seems to me when asked about best fuel mileage, their reply was something such as the minumum RPM at the highest gear. I have no idea if that is or is not true, but it is something I’ve remembered for years.

Of course there's the human toll of all the motor vehicle crashes you'll be responsible for as normal people crash attempting to pass you in frustration.

I remember that 55 MPH thing being highly promoted in the 1970s during the "oil crisis" or whatever it was called. I too young for it to matter much, but I've always remembered it.

Unfortunately I tend to live in the moment when driving somewhere. So that math -- $120 a year -- calculates out to something like 20 cents on this ride I'm doing right now, and in that moment 20 cents doesn't really seem like anything.

I tend to be the same way with gas prices, which fluctuate quite a bit up here in Canada. So news comes out that gas is going up by 10 cents a litre (about 40 cents a gallon) so everyone will jump in their cars and drive all over the place looking for a station that hasn't put its price up yet. But I'm thinking... 10 cents a litre in a 40-litre tank (which is really just a 30 litre fill-up) adds up to three bucks. Am I really going to waste an hour plus all that driving around to save three bucks? Nope.

'Attention waning'
A story that might be interesting to you Mike.
Early in my career in respiratory medicine, including the new field of sleep apnoea, was the story (from the treating physician) of a professional racing driver who had no trouble on the track but was dangerously sleepy on the drive home. It was just too slow to get the 'adrenaline' going and keep him alert. (In this condition sleepiness is not the same as tiredness). Wishing you a great year ahead.

On my bicycle, I get 100 miles per gallon of water.

The truth is, if it was your nature to drive 55 you’d already be doing it. Most people slow down after receiving a speeding ticket or relentlessly check their mirrors after being rear ended at a stop light but this behavior eventually fades.

You seem to enjoy driving and have owned a roadster so I predict you will be back to 65 MPH in a few months. Speed is fun. Drive it like ya stole it.

“I always liked speed” - Guy Lafleur

“Favorite Bat-gadget was probably the Batboat, because it was fun to get out on the ocean and run that thing at high speed.” - Adam West





Because I live in Nebraska, not the Finger Lakes; where I'd cruise a Miata with the top down, at a leisurely pace, enjoying the scenery. On I-80, with a 75MPH speed limit, it's cruise control @ 84MPH, per the "Nine you're fine; ten you're mine" rule of speed enforcement.

Discard that vow of prudence if you ever find yourself driving into Atlanta. This is the only place I’ve ever lived where the speed limit keeps going UP as you get closer and closer to downtown. Where it tops out at 65 as a legal limit, the mandatory minimum for self-preservation is 70 except during the gridlock of rush hour. At all other times 75mph might be wiser — and know that at any instant someone just barely ahead of you will cross four lanes, left to right, without signalling and with complete disregard for all other traffic. Just so you know...

I have recently joined the "Prius Nation" after someone destroyed my old hybrid SUV. I leaped from the current insanity of SUV's & huge pickups, a state of mind I thought we were getting over with, but has once again been fueled (pun intended) by low gas prices. All the current climate reports reinforce my feelings and conscious that I've made the right decision. Are their downsides? Unfortunately my little Prius C is not going to be as good a "photo camp mobile" as my larger hybrid was. "Get over it" seems the appropriate phrase :-)

If your driving involves a fair bit of stopping and starting, or going up and down hills, then the weight of your car becomes a very significant factor in fuel economy, as it’s very costly to accelerate and decelerate or raise greater mass. Accelerating that mass less each time is much cheaper.

If your car has a fuel usage indicator it may also show your average speed, as mine does. That average speed is soberingly low, and almost impossible to raise by aggressive driving short of craziness.

I use a monitoring app to gain insurance discounts. After getting used to driving the speed limit, I realized that the time losses on short drives are miniscule, especially compared with fuel savings and relaxation. What drivers do behind you is their responsibility entirely if you’re doing the speed limit or driving to conditions.

Going fast may cost you some money but what it costs the environment and wildlife in extra pollution is much more serious.

Just because some costs are externalized doesn't mean that they should be ignored. Others are paying the price.

Other factors, such as the road's condition, traffic (slowing down and speeding up often), and tire pressure impact your mpg at least as much as your top speed. For me, driving a 5-speed manual transmission VW Jetta, I get far better mpg at highway speeds - around 70 mph - than I do at slower speeds or in city driving (the opposite of my wife's hybrid vehicle).

Beyond that, you may also wish to consider the time wasted going slower (especially for longer distances) vs. safe speeds for the conditions and the environmental impact of inefficient driving.

I recall that during WWII, the 55 mph limit was set for federal highways in order to save wear on tires, rather than for gasoline usage.

Where I live (suburbs of Indianapolis) I take Interstates all over town, at 65-80 mph. Any slower and you'll get murdered.

When I drive to places around the state, however, I reach for the old two-lane highways every chance I get. They're slower and far less traveled, meaning I get there far less stressed out. If I'm saving gas, that's a bonus.

I sit here with the 81 VW diesel pickup. Stuck with 52mpg as my high on the highway and 46 as my low driving around town. Haul wood and junk in it, put on the topper for camping and protecting the load and it just drives on and on. I don't really look for a fuel station til 600 miles into the tank.
Not much worry about speeds past 70-75 due to gearing but it cruises along just fine at highway speeds. The 13 inch tires are inexpensive, even the top brands and with the lightweight they last a long time.
One nice Photo vehicle. I can even run it on peanut oil, sunflower oil and in a pinch, Auto Transmission fluid.
Sure wish VW would bring the new diesel pickups from Europe into the US. Much better mileage than mine - then again, mine was paid for 33 years ago.
As for newer Electric cars -

I have two widely different vehicles with proportional differences in fuel consumption. First is a 1988 LandCruiser 4x4 off roader. It loves cruising speeds of 45-55mph, and abhors 60+mph. It's domain consists of two lane roads and country back roads, which suits me fine. Fuel use rises as speed increases to 65. It's my go to vehicle for accessing photographic locations.

My Subaru Outback Turbo has an unexpected behavior with regard to fuel consumption and speed. The turbo's effect is pronounced. In Colorado at 5000 to 11,000 feet elevation optimum fuel mileage is 75mph (where the turbo is working moderately for the ideal torque in 5th gear). That's where the torque curve crosses the air density curve. Gas use at 75mph is 20% less than at 55mph. No kidding. My son's Subaru WRX STi acts similarly.

That 55 mph figure can't possibly be true for EVERY car, can it? Sleek new car, beat-up old clunker, gas, diesel, electric, hybrid, huge car, tiny car, FWD, RWD, AWD, 4WD, etc., and under any road condition?

Until November 27 your advice would have made a lot of sense to me, but not so much now.
I had an accident last September and my car was totalled. The money I got from insurance was too short for a new car, so I thought if buying second-hand was mandatory, I'd be better off getting a car that didn't depreciate too much, so I ended up buying an extremely well preserved 1991 Lancia Delta.
The car has the aerodynamics of a truck, the 1.301cc Lampredi engine is carburettor-fed, cold-starting the engine is a nightmare and unassisted steering means parking is the equivalent of a tough gym session, but that chassis was the foundation of the highly successful Lancia Delta 4WD that dominated the rally world championships between 1987 and 1991. And it shows: my car handles like a dream. I'm still shocked at the speed it can carry into tight corners, especially on mountain roads. This beast was made for driving pleasure.
It might be an old car by today's standards, but its cornering abilities were head and shoulders above its competition, and it handles better than many contemporary cars. But it's just 78 HP for ca. 950 Kg. To get the most of the car I have to drive it hard - normally a gear below what I was used to. It's an inefficient, expensive and quite pollutive way of driving, but it never fails to put a smile on my face. I mean a big, ear-to-ear smile. With the occasional "wo-hoo!" scream.
Of course I don't drive like that all the time. When I do, though, efficiency considerations are far from my mind. It's an irrational approach to driving, and it's wrong on many levels, but I just want to have some fun driving a capable, characterful car. They don't make too many of them nowadays, you know.
Silly me: out of Europe, only the cognoscenti have an idea of what I'm talking about. Here's a Lancia Delta exactly like mine: https://img3.annuncicdn.it/12/01/1201ae4119896738f51c2b38e4def864_orig.jpg

I went to a Lyle Lovett concert once and he said he flew into Amarillo, Texas, on his way to Lubbock for a concert, and he asked the guy who picked him up exactly how far it was to the concert site. The guy said, "A hundred and fourteen miles -- 'bout an hour."

When I drive from Santa Fe down I-25 to the airport in Albuquerque, which I probably do about once a month, I set my cruise control at 85. The speed limit is 75, though nobody but people from Oregon and Vermont, driving Priuses, and probably towing tanks of ****in' maple syrup, observes that. I was once cruising along at 85 listening to a few tunes and playing the drums on the steering wheel when a cop snuck up behind me...and then passed me, continuing on his merry way to Dunkin' Donuts or something.

The speed limit on I-10 east of El Paso is 80mph, but nobody drives that.

So, just to complicate this, there is a speedo variance in almost every car or motorcycle I’ve used that amounts to a reading 3-5 mph higher than your actual speed.

This has been confirmed with Garmin units as well as Waze on my phone, and is proportional to the speed range. That 3-5 number is at highway speeds.

I have read that the manufacturers intend it that way to add a small buffer to help avoid tickets. Whether that is correct or not I can’t say, but I have been aware of this for over 10 years (I owned over 30 cars in that time for work and pleasure) and have never seen a speedo reading too low, so maybe there is something to it.

If it was random assembly line variance, some would be high, some low, some accurate. But none are ever accurate or low.

What does that add to the conversation?

1- If your speedo is saying you are going 65, maybe it is really 61 and that’s why the police are ignoring you. That’s not a conclusion, just a possibility to consider.

2- I don’t like using the trip computer built into the car for this sort of thing, because I don’t trust it. I’d rather use actual miles driven divided by gallons used, and average that over several tanks assuming the driving is the same sort tank to tank, highway, local, whatever.

As always, it depends...

If you have a normally aspirated engine, sure, that might be correct.

Many modern cars in the last 5-10 years are turbocharged, and going faster actually does not introduce significant fuel consumption due to the fact that "waste" exhaust gases are re-circulated, improving overall efficiency when the turbo is engaged. Therefore, going say, ~75-80 mph (120-130 kph-ish) might actually be more efficient overall than 55 mph ( - both in terms of fuel efficiency *and* time (confirmed by myself in annual 2000 mile identical road trips and tracking fuel consumption with live data via an app connected to a car scanner dongle). True, I do achieve better mileage at 55 mph, but the time saved far outweighs the fuel saved (e.g. 15% fuel savings versus > 40% time, so you might be saving fuel instantaneously but the duration of the trip causes you to use more absolutely [What? No, not so--miles per gallon doesn't have any time factor. --Mike]). Combined with modern aerodynamics and low resistance tires, it's not nearly as bad of a mileage hit as you'd expect with "older" tech. (Bringing it back to photography, maybe analogous to photon efficiency of newer sensors, despite having smaller area compared to lower res bigger photosites, I guess?)

Mind you, around town it's worse, but not so bad if you can use a light foot and stay out of the turbo range. Though that takes the fun out of driving a turbo...

Now, how many hours of your life are you giving up to save that $200? Does this really make sense?

I have seen many school math tasks based on square dependency between speed and fuel consumption. Also saw people confirming this from their experience, but I cannot confirm this myself. I have also read that when you exceed some limit, depending on your vehicle, the aerodynamic drag introduces ***cubic*** dependency. But the final result is of course a combination depending on many factors.

There's more to life than gas mileage.

There's a lot of economics in this calculation. What we gain in fuel efficiency may well be eaten up by the other inefficiencies caused by taking longer to get places. Optimizing trip characteristics is multi-dimensional and fuel efficiency is only one dimension. For an individual going to dinner, it probably doesn't matter much--their time has a low cost. But for those who drive for a living, it can matter quite a bit.

The design, condition, and demand of the roads in question has a large role in this calculation. In the west, the marginal benefit of a higher speed limit over a long trip can have a substantial impact on the trip, making it possible to get there in a day rather than two, or four days rather than five (which saves lodging costs). For commercial drivers who are subject to time limits each day, an arbitrarily low speed limit can significantly increase their costs, and exacerbate the already severe problem of having a place to park during their required rest times. The dissolution of rural towns has only made this problem worse.

It seems to me that those who are attempting to drive faster or slower than the prevailing speed on a road will be the ones who disrupt the smooth flow of traffic. That can have consequences, too. It may not be the fault of a slow driver when a faster driver passes unsafely, but if the slower driver is driving below the prevailing speed, their behavior may still play some role in the outcomes. What we used to call defensive driving meant causing the least disruption in the traffic stream.

All that said, some of the speed limits I've seen in Texas recently surprise me--they are higher than what most prudent drivers would be comfortable driving on those sorts of roadways, and higher than they would have been at any time in the past. The problem is that it's been a long time since we expected drivers to choose their own speeds based on prudence, and the skills remain undeveloped. So they drive at speeds uncomfortable to them, and get into trouble quite separate from fuel efficiency. Speed prudence needs better treatment in driver training and retraining programs.

Back to you, Mike. Would you notice that $200? I suspect it would be about as insignificant, in the increments you would see it, as the extra time required to drive 16 miles to dinner. That problem confounds economic analysis on a large scale--the costs associated with road use are usually simplified to the point of irrelevance for individuals.

When I went from SF to NYC on a Zero DSR electric motorcycle in 2016, I spent a total of $42.73 in charging fees to travel 3,682 miles. That's $.0116 per mile. My Honda S2000 is likely the last gas-powered car that I will own.

Far safer to drive with the flow of traffic, whatever that speed may be. Alone on the highway, or the uncongested two-lanes of rural NY State, set your speed as you wish.

As a former participant in motor sports, I spent a lot of time with people who think we should be allowed to go faster on our roads. Sometimes that makes sense, and sometimes it doesn't is about the most you can say. Doing 55 mph (roughly 90 kph) on a lonely deserted stretch of highway seems nuts to most people, who are willing to trade off gasoline mileage with alertness level, boredom, time constraints, etc. People instinctively figure out how to optimize these things. So a speed limit of 70 mph (or 110 to 120 kph) on an open road seems ok to me. If you want to drive more slowly, that's ok too.

But our insistence at trying to maintain these maximums within city limits is profoundly stupid. In general, we are not trained to drive nose to tail at 70 mph in dense traffic, no matter how many NASCAR races we watch on TV. When I was in Finland years ago, where they take driving seriously enough to actually properly train and test drivers (we don't), limits within Helsinki environs was a very strictly enforced 80 kph but once you were out in the boonies, the max was 120 kph. What could make more sense?

People are unpredictable and nervous. If you try to do 70 while some old lady is trying to do 55 and you get mad at her, you should chill and go back to driving school. Public highways don't belong to you. That old lady has as much right as you to be there, maybe more if she's been paying taxes longer. Highways are public thoroughfares, not testosterone proving grounds. If you really think you're a good fast driver, buy/rent a race car and go to your local track and leave your ego there.

There seem to be a lot of people who think that they are more important, so their time is more valuable. They are mistaken.

If you do the math, you'll see that it doesn't make sense to speed for short trips (+/- 20 miles). Purely considering arrival time. For longer trips, see Kirk Tuck's post. . .

Mike, depending on your hourly earnings rate, the time saved may generate enough to more than offset for the added cost in gasoline.

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