About 52-55 mph seems to be the optimal speed for gas mileage for my car. Since my 2008 Prius driven in such a fashion averages about 52 mpg in the summer, I think I sort of know what I’m talking about–at least for this vehicle.
An engine that is lugging is not likely going to be getting its best fuel economy either.
Regarding keeping up with traffic, a big factor is consistency of speed. "Keeping up with traffic often results in participating in the same “standing wave” behavior as other motorists, so you end up accelerating and decellorating or braking (worse again).
I set cruise just above the limit and pull into the inside lane…
I thought the 55 MPH speed limit stunk when it was the law. My opinion of it now is no higher. I’m just glad those days are over!
I know of nobody outside of the DC Beltway that did NOT think the 55 mph limit stunk when it was imposed. And I know of nobody whose opinion has since changed.
The internal friction from the powertrain and the rolling resistance from the tires is pretty much a linear function so they do not change the fuel mileage according to speed. Lower gears have higher friction though.
Areodynamic drag is non linear and increases at some exponential rate as speed increases. In theory anyway, the optimum fuel economy should be the lowest speed in the highest gear. Various cam profiles and other engine design elements have an effect on this though
I think that you will find that most vehicles have a “tipping point” or “knee of the curve”. That means that the mileage will not be affected greatly as speed increase, just small decreases with rising speed until the tipping point is reached. Above that speed, gas mileage drops much faster with small increases in speed.
For example, my 97 Nissan Pick Up gets about 25.5 mpg at speeds up to about 60 mph. In the 60-65 range, it still gets 25, but at 65-70, its down to 24, 70-75 sees about 22 and 75-80, the mileage drops to 18. Never tested it above 80, and only once at the 75-80 range (75 speed limit). My Saturn on the other hand doesn’t seem to see much of a drop as speeds approach 80.
Quoting @genex “It never was an optimum speed , more a politically motivated one that got revoked when enough people got angry .”
Luckily the majority of 535 of those angry people turned out to be our federal legislators.
It’s just like the federally mandated seatbelt interlock law of 1973 & '74. Once congressmen (or more likely their wives) got fed up with having to buckle up to make their brand new cars start, the law got repealed.
Wow Keith that was the same mileage I got with my nissan pickups and other 4cyl vehicles,my Dakota gets best mileage around 45 mph,everybody else around here with V-8 Dakotas claim better mileage then I get-I say up to a certain point slower is better-Kevin
Get a friend to drive beside you at 55 mph on any interstate or open road and see how long before someone is pushing you and questioning your ancestry.
That was you??
Maybe drive in the right lane, behind each other from now on.
It depends on what you mean by optimal. If you mean the best miles per gallon, lower speeds rule. Driving at 55 is better than 65. And 45 is better than 55. It depends on the car, but it’s hard to win much beyond the basic physics. Slower is better until you reach 30 mph or so, when other factors start to take over. The reason speed is so important is that the effect of wind resistance on mpg goes as velocity squared. So a small increase in velocity yields a big increase in wind resistance effects.
MPG isn’t the only factor of course. Driving at roughly the same speed as the other traffic is safer than driving at 30 mph when everyone else is going 60 mph. So if what you are looking for is the optimal safe speed, probably somewhere around 55 mpg is a reasonable guess.
From wikipedia:
"the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph … requires about one-third the power to overcome wind resistance, or about one-half the energy per unit distance … "
I remember well the 1949 Nash AirFlyte 600. The 600 model designation came from a road test where this Nash went 600 miles on a 20 gallon tank of gasoline. I would guess that the highway cruising speed for this test was probably 50 mph, and that Nash was equipped with the optional Borg-Warner overdrive.
I don’t think the highway mileage could be attributed to an efficient engine as much as an aerodynamic design. The engine was flat head 6 of 172 cubic inch displacement and 82 horsepower. The body was designed in a wind tunnel and had about half the coefficient of drag as competing cars. The Nash 600 had a top speed of about 78 miles per hour which let it keep pace with other cars of this period with larger engines but a less aerodynamic shape.
As an example of the difference the shape makes, the 1949 Studebaker 1/2 ton pickup had roughly the same size engine–a 169 cubic inch flathead 6. The engine was screaming for mercy at any speed over 45 mph. While the truck did weigh more, the real problem was its shape.
I remember when my parents had an old Nash @Triedaq. It was a 40’s, but I don’t recall the model. They bought it used, and it was pretty “used” when they bought it. But it was reliable even so. And like you say it got good gas mpg. I always liked that car. It had a cool look to it. Shiny, black, and sleek. It had cool controls and dashboard too. I think you started it by pressing a button on the floor with your foot.
The thing I most remember about that ol’ Nash is that it would backfire for any or no reason. Especially when the engine was cold and the car was going uphill. When my parents would drive away from the house the first thing in the morning, it was up a hill, and everybody in the neighborhood woud hear “bang” … “bang” … “bang” … really loud, and they knew my parents were on their way to work.
Backfires. What a life!
Well I lose 2mpg driving at 75 vs 55, your call.
Tom and Ray once answered this very question, and they claimed the best mileage was, as someone posted here, after the transmission locks in its highest gear, which is commonly in the low 40’s. Of course, if you start going up hills, that may well change.
I agree it was political non-sense. Go ahead and drive 40 mph if you want but my time is worth something. Back in the days of the 55, I had to drive 100 miles a day at low speed thanks to Tricky Dick and the highway patrols loved it.
55 MPH may be a more optimal driving speed than say, 70, for gas mileage. But definitely not for sanity. I hope we can put that long cold period of forced ‘double-nickel’ speed limits to rest forever, much like the cold war of the same era. I will cheerfully pay the difference to go faster, even if gas makes it up to $6/gallon…