Fuel-Optimal Speed = 44mph for all cars?

44 is a generalization, but it is fairly consistent for non-hybrids.

47 to 49 MPH seems to be the most economical.
This pic was taken 30 miles after a fill-up of pure gas.
http://i139.photobucket.com/albums/q297/spokybob/P1010051.jp...

My highway efficiency is the same at 45mph as it is 70 mph on level highways such as I-80 across Nebraska or highway 400 - 52 across Kansas and the wind is under 25mph blowing towards me. It goes down with the increase in speed from 42-43mpg to 39-41mpg at 80mph.
Your 99 efficiency driving would be a pain if you had to drive a long distance for the interstates would be off limits so one would have to weight the differences on certain trips.
One would also have to compare the numbers with the air conditioner is running or not and how fast is the fan running. I did notice the difference in mileage while driving around Phoenix in 117 degree heat and 75 degree heat.
My hybrid has over 100,000 miles on it and at the last oil change they replaced my water pump belt because it was very frayed. My city driving went from 42-43 mpg to 46.5 mpg my last 67 miles on this tank.
One cannot compare cars for everyone drives differently from jackrabbit starts to the next stop light and then waiting, to having an egg under the foot on the gas peddle and meeting the other at the light

I don't know but I suspect I'd get pushed off the road if I tried to maximize mileage - heck they push me when I'm at the speed limit on an inside lane - not the passing lanes !

Here are some links and they seem consistent that the "sweet spot" is 50 to 60 MPH.

http://eartheasy.com/move_fuel_efficient_driving.html

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/q...

http://www.fueleconomy.gov/feg/drivehabits.shtml

http://www.mpgforspeed.com/

Which has a V-6 , I've figured somewhere about 57-58 mph gives me best mileage. I really only see this when driving on an Interstate in a construction area where the speed limit is reduced to 55 mph.

I've also noticed with the V-6 that my mileage starts to drop a couple of mpg per 5 mph increase above 65 mph.

I typically can get about 30 mpg at a constant 70 mph.

4 cyl fwd auto 8L/100 KM, driving @100-120KPH or 29 US MPG, driving @ 60-75 MPH in 0-6 C/32-50 F

My 265WT used to show the best score of 99 on ecoRoute while traveling at 55 MPH. But some time ago, it changed and now shows best at 65 mph. I assume it was with an update as I have not changed any settings in the nuvi.

Likes 57-59 MPH windows up cruise on AC off 33.5 MPG

I believe, based on this thread and much other chatter, that most people fool themselves into thinking their fuel economy optimum is much faster than the truth. I suspect that for most cars the truth is that it gets better right down to the slowest you are willing to go in top gear. Going to a lower gear hurts, but the air resistance component, for which the dominant term is square law in air speed, wins out over the loss in engine efficiency from running it at lower than optimum thermal efficiency.

For my own Audi 2002 A4 I spent quite a bit of time gathering some consistent data. This graph compares fuel economy at 30,40,50,60,70 mph on a hot summer day with the air conditioning on:
http://i6.photobucket.com/albums/y231/PeterAStoll/problems/a...

As you can see, it got steadily better right down to 30 mph--which is OK in fifth gear on this manual box car.

And this table compares a few windows up/down aircon on-off conditions at a smaller set of speeds. I took the data in the same car.

http://i6.photobucket.com/albums/y231/PeterAStoll/problems/a...

Folks who believe they are saving fuel by using aircon in preference to window opening are, quite generally, wrong. I believe. Using aircon for comfort, is, of course, another matter.

I do agree that the square law air resistance thing means that for a sufficiently high speed aircon would be better on most cars if they could go that fast, but for my Audi that speed is far above any legal limit in the USA, and quite possibly faster than the car actually can go.

I'm curious as to how your data was obtained (and my newest car is a 1995 Olds Ciera, so I'm not familiar with how modern cars with a fuel economy gauge might determine fuel economy).

Did you do something like run a tank of gas at each of those speeds and conditions?

Or did you take readings off some sort of fuel economy meter?

If so, does the fuel economy meter in your car actually measure (somehow) the actual gas used in various conditions?

(I assume the latter / last, and I would think that in modern cars with fuel injection there would be a variety of simple ways to inexpensively determine actual gas flow, like measuring the amount of time the fuel injectors are open or things along those lines.)

You have to figure out what RPM is the best for optimum fuel economy, and work from there. My last was was a 2010 VW Golf diesel with a 6 speed manual, and I know from watching my fuel economy gauge it got better mileage at 55 in 5th than in 6th. This was because in 6th the engine was running at too low an RPM, and was off its peak which was about 1700-1800 rpm.

Optimal speed for my V6 Camaro is 57.

Friction, drag, and the engine are all factors in fuel efficiency and the sweet spot that the engine likes to run for best fuel consumption. The optimum RPM for my TDi is about 1700-1800 RPM, which turns out to be 50-55 MPH. That is from dyno data and hundreds of thousands of miles and record keeping.

The engine itself is probably the biggest influencing factor and how it is designed. An engine for general use is designed for an extremely wide range of operating conditions, trying to be all things to all at all times. Never a good thing for optimizing fuel efficiency.

Sunday I took a 260 mile round trip in my hybrid with one way against a wind that unfurled those huge US flags along the interstate. At 78mph average I got 41 mpg and on the return trip I got 43.3 mpg with the wind behind me. Not much difference. I could have gone faster but all it would have done is having me tailgate all those other cars driving slower.
In my city there are two streets that I frequently drive with the speed limit 35mph. I try to see how long I can drive them not using any gas and I can do the 3.4 miles with the gauge reading 99.9mpg (it doesn't go higher).
My optimal speed is 35mph on a level street where I do not use gas at all and pretend there is an egg under my gas peddle foot.
Toyota every year in our city has a race to see who can drive a given course in a given time with the best gas mileage. The winners in the last two years did over 75 mpg and they were elderly ladies. Women drivers bah.

There are many after market tools or ask your auto mechanic to tune your car for a better fuel efficiency at higher or lower speed depends on your driving style.

There are many factors that govern at which speed a vehicle will operate the most efficiently.

Thus specifying a "one size fits all approach" is useless.

The score is just an approximation which then can only be used as a guide.

In other words not only is it not accurate for one vehicle but also not to be applied for other vehicles.