mpg "curve"

[NutmegCT]

mpg "curve"

The sad sad remnant of my brain does manage to recall that, generally speaking, the faster I drive, the lower my mpg.

Is there a way to figure where the dropoff occurs for a specific engine?

My boring daily-driver is a *great* 2000 Mazda Protege LX, 1.6 liter, 5 speed, regular gas. Gets 38mpg, which hasn't changed since I bought the car in March 2000. Mostly highway driving at 60-65, altho' I try to move at traffic speed when traffic gets heavy.

From the US Fuel Economy website:

<span style="font-style: italic">"While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.20 per gallon for gas."</span>

I realize the above is very general. How can I determine the "optimal fuel economy speed" for my Mazda? Is there some "optimal speed" chart for different engines and/or cars?

Thanks.
Tom

 

[Mickey Richaud]

Re: mpg "curve"

Is there a way to figure where the dropoff occurs for a specific engine?

My TR8's point is when I start the engine! :wink:

Good question, though, Tom. I'll be interested to see what folks have to say.

Mickey

 

[Twosheds]

Re: mpg "curve"

Remember that drag increases with the square of velocity. The drag at 60 MPH is four times the drag at 30 MPH.

I think so, anyway. Not much left of my brain either.

 

[Basil]

Re: mpg "curve"

When I drive across country (Albuquerque to Huntsville, AL) in my C5, some states (like New Mexico) have 75MPH speed limits, while others (like Texas) have 65 MPH limits. At least for that car, my mileage is about the same at either speed - 31-32 MPG.

 

[Brooklands]

Re: mpg "curve"

I like to use gallons per hour. The faster I drive, the better my ratio. And if the gauge is low, speed up so that you can get to a pump in just a few minutes to prevent running out of gas.

(It is kind of like turning up the radio to solve a bad mechanical sound in the daily driver car)

 

[aeronca65t]

Re: mpg "curve"

Yes. John H. is correct.

There are, of course, a ton of other factors and they even differ from car to car. I doubt there is "one curve" that would represent all cars.

I would guess that the most efficient "sweet spot" for going the greatest distance on a certain volume of fuel is about 25 to 35 mph in most cars.

This does not take into account the factor of "time", which has great value in many cases (would we really want an ambulance trying to acheieve best mileage, for example?)

As the car goes into higher speeds, parasitic losses from internal engine friction, gearbox, tires, aerodynamics etc. probably increase to a point where they may offset other efficienies, such as spark advance, cylinder scavanging, etc.

By the way, one of the highest areas for parasitic loss is caused by the engine sucking against the throttle plate. Since bigger engines tend to run at cruise with the throttle "just cracked open", they inherently waste more energy than small engines. Many diesel engines are unthrottled, controlling only fuel.....this is part of the reason they are efficient.
In a perfect world, the most efficient engine would designed so that the throttles would be wide open at cruise. This assumes low restriction port and manifold design too.

Most cars get about 25% thermal efficiency for gasoline on a really good day. This means that 75% of the energy we put in the gas tank is wasted (or at least, not used).

 

[Tinster]

Re: mpg "curve"

Tres Gatos is best at 2800 rpm, on plane,
at about 24 knots.

d

 

[kennypinkerton]

Re: mpg "curve"

You get drag losses (as described above), how much extra weight (junk) is in the trunk or the back seat, is the car washed and clean or dirty, when was the last oil change, with what weight oil, are you following 500ft or 100 ft behind that eighteen wheeler, does the car cruise at 1500rpm or 3500 rpm, overall, how efficient is the transmission, the final drive, etc, etc, etc. And all of that info is going to be different for each and every car (even if it's the same make and model).

 

[NutmegCT]

Re: mpg "curve"

lots to think about here ...

is there a way to figure the most fuel-efficient mph for cruising?

the generic "fuel economy curve" from DOE shows a slight increase in mpg from 35 to 55mph, then a slight dropoff after 55.

just wondered if there's a way to figure that for my own car - in other words, for the 1.6 liter, 5-speed in my Protege.

Or ... for all practical purposes ... should we just consider that generic DOE curve applicable? (altho' DOE doesn't give references for the curve itself)

Thanks.
Tom

 

[PAUL161]

Re: mpg "curve"

With all the figures illustrated in the above posts, I don't see anything about frontal compression. The more easily a vehicle,(car, airplane), slides through the air, the less frontal compression, thus, the less power it takes to push the vehicle through it. The DOEs 55 MPH specs are generalized and though close, their not necessarily true for your vehicle. The newer vehicles with aerodynamically designed bodies, more fuel efficiant engines and some with lower RPMs per mile might be in the 65 to 70 MPH range.

 

[angelfj1]

Re: mpg "curve"

Except for the wedgie crowd and perhaps spits, I would think that most TR's have LOTS of frontal comperssion!!!

 

[RonMacPherson]

Re: mpg "curve"

Remember reading a book on auto aerodynamics years ago. It stated that the horsepower/aerodynamic crossover point was 48 mph or so. That above that speed the horsepower required to increase 5 mph started geometrically increasing. That's why slow beetles were so fuel efficient.

 

[texas_bugeye]

Re: mpg "curve"

My BE starts dropping MPG about the time it starts to sound good.

 

[leecreek]

Re: mpg "curve"

Once they 'get on the cam' they are pure music.

 

[texas_bugeye]

Re: mpg "curve"

I rate mine SPM Simles Per Mile.
Something thats has amazed me is since I have moved to Texas is the affect wind does have on MPG.
I have been driving to and from Dallas and Odessa for a few weeks now Cruise on at 70 all the way there and back. Basic flat four lane interstate. Depending on the wind dicrection an strengh with a villager van with 240,000 miles on it I can get between 19 and 27 miles per gallon! First trip or two a thiyght something has to be a miss but the only real variable was the wind. THe cross sction on a villager van is not that huge comon its a mini van with a fairly wedged shape front end. this is about a 300 mile trip.

 

[Basil]

Re: mpg "curve"

I rate mine SPM <span style="font-weight: bold">Simles</span> Per Mile.

Are you sure you don't mean "slimes" per mile?

 

[Stewart]

Re: mpg "curve"

Your right on the wind. There have been days when I've gotten 4mpg one way and 25 going to the other. Freaking north dakota.

Going to be interesting to see what the milage of my 73b is. I went down to fargo and topped off the tank and it was at 21mpg going 75 with a good head wind. On the way back I took the slower country roads at 55 to 60mph as I wanted to listen to car talk and the gas guage is still pegged at the full stop. I should of however taken the interstate as I got caught in a deluge for about 5 miles and while the windshield wipers work great I need a second set for the inside of the windshield.

 

[texas_bugeye]

Re: mpg "curve"

Ah yes Dakota.. I lived in Minot Going thru high school (airforce brat) When we got off the plane the day we were stationed there I asked dad who he yeld at and was firmly rapped up side the head. Why not Minot? freezin is the reason! I don't mind not going thru any more winters.
they were long and rough.
Great place to check MPG looong and flat roads.

 

[PC]

Re: mpg "curve"

... just wondered if there's a way to figure that for my own car ...

There's only one way, measure it. “Typical” curves tend to be wildly inaccurate when it comes to things that vary as much as cars do from one to the next.

Manufacturers and researchers use specialized sensors combined with sophisticated data acquisition (DAQ) systems to collect that sort of information.

Modern technical advances (laptop computers, etc) have made DAQ equipment remarkably affordable compared to the laboratory gear of yesterday. But it still may be too extreme (and expensive) for the vast majority of hobbyists.

Luckily (or unluckily depending on your point of view), all new cars are jam-packed with sensors. And a lot of that sensor data is available because of government mandates.

Since 1996 all new cars sold in the US conform to the OBD II (onboard diagnostics) protocol. Besides the cryptic failure codes and the dreaded “check engine light,” sensor data is also available over the OBD II link.

Here’s an outfit that makes gear to read sensor data: https://www.scangauge.com/


PC.

 

[DNK]

Re: mpg "curve"

Except for the wedgie crowd and perhaps spits, I would think that most TR's have LOTS of frontal comperssion!!!

Hey Doesn't Doc have that?