These threads are so funny.

 

My sis-in-law has driven two Prius's. No frills, to be sure. But to be honest, I was quite impressed with the interior room. My 6 foot frame fit well within the cabin, with head/foot room to spare. In Seattle traffic it got 49mpg all week long.

 

yeah in traffic they get good mileage but it doesnt do as well on the open freeway or atleast my wifes didnt. Hers was one of the older ones though (2008) i think. Personally i just always felt cramped in them. Ofcourse i feel cramped in her sorento and it technically has more driver room thrn my e250.

 

My best best best ever mileage I ever recorded on my E400, was after my wheel/rim got bent on that stupid PAID highway going back from Bali to Jakarta in 2021.
I had to go slow at no faster than 100KM/H as vibration from the bent wheel increases past 100KM/H ( 60 MPH).
I recalled I got an insane 12KM per liter or 28 MPG.
My average in town driving is 7KM per liter or 16.46 MPG
My average out of town long distance at best is 9-10KM per liter or 21.16 to 23.52 MPG with lots of 6,000 RPM WOT and pass 100MPH when road is clear.

When I was a kid, my family owns W126 280SE manual and Audi CD100 2.2L 5 cylinder also manual, this was in 1983. This was in Singapore, not Indonesia.
This Audi : https://en.wikipedia.org/wiki/Audi_1..._2.0_Front.jpg
Me parents bought me a Supra 2.8L 6 cylinder manual, Mk2/A60. this one , not facelift one with nicer wheel and fender : https://en.wikipedia.org/wiki/Toyota...e_pub_1982.jpg
The way I drive the W126 and my Supra is the same, 4KM per liter 9.4 MPG if RPM doesn't hit WOT, I dont feel good.

So when the Audi CD100* arrived brand new ( *cheap and garbage car at that time ) replacing the family Saab 900 Turbo ( I ruined this Saab engine and tranny ..LOL ), this one : https://en.wikipedia.org/wiki/Saab_9..._2.0_Front.jpg ,
naturally I took the chance to know the Audi CD100 by driving her a lot and do top speed , skid her and yada yada. Handling is shiet, massive under-steer. But its wiper is the best in the world at that time I guess and probably still is if I compared
to all cars I ever experience their wipers. It was very aerodynamic at 0.30 drag coefficient which was awesome for the early 80s and the wiper has the spoiler, that is why when I top speed her in the rain, wiper still works and does not get lifted up
like in my Supra pass 110MPH in the rain. The Audi small tires was 185 width only, imagine so skinny and hence in the rain pass 100MPH I do not aquaplane as often as my Supra with 205/225 front rear.

From full tank to the first 400KM of use, I thought I had a defective fuel gauge on the Audi as it was only a tiny bit under 1/2. Fuel tank should be near empty at 400KM.
I brought her to Audi workshop and ask the tech, how the hell the fuel gauge does not go down much ?
He told me, well the fuel tank is quite big 80 liters...wow .. okey but fuel gauge should read 1/4 at least. Assuming 400KM if 6 KM per liter = 66 liters gone.
So I went to top her fuel up and done the math, holy cow this Audi 100 can do almost 10 KM per liter !!!! or 23.52 MPG
The Audi has 5 speed tranny, my W126 only has 4 speed I believe. The 5 cylinder is rough. But this is the most fuel efficient 2 to 2.8 liter car I ever driven, with my style of driving.

So that was my experience with fuel economy.

At least none burn fuel like my co-owned 7.5 ton , small 34 footer boat with 250HP x 3 Suzuki 4 stroke which is 1 to 1.1 mile per gallon typical at cruise speed depending on weather and passenger load.

 

I almost never look at my tachometer and I have several cars with them, including actual sports cars with manual transmissions. Never use the paddle shifters either.
This week I looked to see what RPM I ran at different speeds: 35 MPH-1200; 40-1400; 45-1450; 50-1500; 55-1600; 60-1900. All of these were on level roads using cruise control. Haven't had a chance to try 65, 70 or 75 as have not been on interstates.

Do not know what the RPM has to do with gas mileage, but noticed a couple of threads discussed it.

 

I logically follows that the more crankcase revs the engine does per minute, the more times the spark plugs fire per minute, ergo more fuel used.

My C320 (3.2L V6) makes 2950 rpm at a steady 70mph, because of its fewer overdrive gears (5th is top). My E350 makes much better mpg at the same speed (1950 rpm in 7th gear), meaning fewer gears to maintain the same speed (70 mph).

 

Try this : https://x-engineer.org/brake-specifi...sumption-bsfc/


I use to plot fuel efficiency table for yachts on long distance journey to calculate maximum fuel range.
I filled up fuel a 100% first, and do 1 hour sea trial at various speed and noted the fuel burn Liters Per Hour vs speed at various RPM.



For cars the engine load is very dynamic or very little given a flat road, hence they use dynometer to do the BRAKE.
However, at the least you have some guideline.

"The lowest BSFC is represented by an “island”, usually at mid engine speeds and high torque (load), close to peak full load torque."



On a small boat with 250HP x 1, see here : https://www.suzukimarine.com/boat-te...pro-boats-219/




On boats/yachts we use engine's peak torque RPM range as best fuel economy. However a boat engine is forever loaded.
The propeller size is a fixed transmission ratio ( single speed tranny ) so to speak and the engine load at cruise speed is similar to climbing a 10 degree gradient of a forever climbing hill, so engine load is decently high all the time.

=============
I seen the fuel economy meter on my car 100KM per X liter thingy, never care to use it at all. I bet it is quite accurate because fuel injection QTY is known and speed too.
Perhaps you can use that to do experiment in a nice empty highway at different gears ( manual mode ) and different suitable speed.
=============

Load is a major factor in fuel burn.

I give you an example where RPM stay the same always but load will change, a diesel generator.
Decent size 320Kw unit in USA will be 1,800 RPM to do 60 Hz, in a 50Hz country it will be 1,500 RPM.
Below is a 50Hz version, 320Kw unit, 1,500 RPM Caterpillar.
https://www.avesco-cat.com/fileadmin...pdf/DM2273.pdf

Go to page 4 of the PDF. I done the easy viewing table for it below.


Diesel generator below 40% load is poor for fuel economy on the diesel engine side and also the electrical alternator side. See above, 75% load as best economy.



Here is a true logging of fuel burn on my last yacht delivery Singapore to Jakarta, Leg 1 of the trip.
She uses 2 of these 900HP MAN common rail. https://www.performancediesel.com/wp...cht/V8-900.pdf

By the time we reached destination of Leg 1 of approx 350 nautical* miles (* x1.15 to become statute miles ), she is lighter by approx 2,400 Kg from fuel burn and useage of fresh water.

There is a special condition call HULL SPEED which is a fuel killer for planing ( fast) yacht. Once a hull passed 1.0 hull speed, she burn fuel like crazy. No choice, hydrodynamic force at work.
For 15 meters yacht of 25 tons or so, able to do top speed of 30 knots is like a car able to do 125MPH speed class.


The liter per hour burn is accurate as per the MAN given propeller load curve, the 1,900RPM. Below graph is based on single engine, table above is based on twin engine.







So the summary is, depending on your engine design, where and how you use your engine LOAD ..... is most effecting the fuel economy.
Sure, higher RPM usually means more fuel burn, but in a generator scenario it is a fixed RPM all the time to make sure electrical frequency stay 50 or 60 Hz.
The new electronic governor for diesel generator is awesome, its RPM control can be as good/stable as 3-5 RPM, no joke. Per 30 RPM drop is 1 Hz loss and 1 Hz loss is a super big deal.
1,800 / 30 = 60 Hz and 1,500/30 = 50Hz

How you modulate the throttle also plays a role in fuel economy, not so much different now that our engines are throttle by wire.


For me, unless the highway is super empty or empty enough, doing 60 MPH constant is not easy.
Too many trucks or slow traffic hogging at lower speed even on fastest lane.
A lot of idiots on the road who does not undertand that acceleration is the fuel burner and not speed itself.
Example, the highway around the city of Jakarta. When the road has a tiny bit gradient because of bridges and what not, idiots in small engined cars who would be already at highest gears,
would loose speed from 80KM/H to as slow as 50 KM/H. They do not want to drop gear because they worry more fuel burn. When the road flatten, they then accelerate again to 80KM/H. This is a waste of fuel.
Drop 1 or 2 gears down and maintain RPM is a better fuel burn.
I used to drive a 1.0L caburated Toyota Starlet eearly 90s for a few years and I drop gears all the time to not loose speed at gradient.


Your must understand that non turbo 1.2 to 1.5L are the most common engine size and 85% are manual gears in my city/country.
If you drive a manual gear with such low power engine, you loose speed fast at highest gear if at only 80KM/H when comes a gradient climbing up a flyover.

This accelerate/de-accelerate process is what made city driving having so much fuel burn.