Best cruise speed for mpg?

[lparsons]

I've been wondering what the best speed is for mpg on our 5.0 mustangs? A little more than half my commute is freeway, with speed limits between 55 and 65.

I remember my previous mustang (1978 4cyl, 4spd) actually did its best fuel economy around 70-75 mph. But I haven't taken the time with this one yet to find its mpg "sweet spot". I seem to get about 20 mpg right now whether I'm doing 45 or 85, which is really counterintuitive for a car like mine with 4.10 gears.

Any observations?

 

[Venom351R]

Hard to say, depends on gearing and mods that are done. Some will probably be better then others at the same speed. Regardless from what I have heard the best MPH to drive at and get the most MPG is 55

 

[ronstang94]

I have 2.73 gears and an Aode. I have to get up to 50mph for the transmission to shift, but then I back off to 46mph and lock in the cruise control. The RPMs are just barely over 1000, so that has to be some good gas milage

 

[Pokageek]

"I remember my previous mustang (1978 4cyl, 4spd) actually did its best fuel economy around 70-75 mpg. "

Where do I get one of those!!! 75mpg!!

 

['94ChickStang]

"I remember my previous mustang (1978 4cyl, 4spd) actually did its best fuel economy around 70-75 mpg. "

Where do I get one of those!!! 75mpg!!

Am I wrong or should this thread be more about RPM ranges?

 

[Zero Signal]

I don't know, but my friend's damn LS1 Camaro get's like 33mpg hwy becuase of the 6th gear and the factory gas saver mode. Why can't we have that?

For the stang, I used to get 20-23mpg depending on how easy I took it, so I don't think you'll get much more out of it.

 

[Car Nut]

I get 2-4mpg better mileage dropping from 70 to 50 mph.

Most interstate trips where I stick to 70mph and cruise control net me 24mpg. On some long Georgia backroads, I've done 26+mpg around 50mph on cruise.

 

[1987Rock]

I average 25-27 with cruise set at 70...that's with a 5 speed, 3.73s and 18s

 

[lparsons]

"I remember my previous mustang (1978 4cyl, 4spd) actually did its best fuel economy around 70-75 mpg. "

Where do I get one of those!!! 75mpg!!

That would be proof that indeed spell check cannot save us all.

That or I was driving downhill, with a tail-wind, the entire way. The world may never know...

 

[scooter1]

"I remember my previous mustang (1978 4cyl, 4spd) actually did its best fuel economy around 70-75 mpg. "

Where do I get one of those!!! 75mpg!!

i dont think a ricer would get 75 mpg...lol

 

[lparsons]

Am I wrong or should this thread be more about RPM ranges?

I was pondering that myself.

Though based on my experiences with my previous mustang, I would say that the results could be counter-intuitive. The 78 would of course be turning more RPM at 70 than at 60 (especially since it was a 4 speed standard transmission) but yet get better mileage at the higher speed.

So perhaps the engine actually is operating more efficiently at slightly higher speeds?

Which is why I've been wondering what other people have observed. I probably need to drive with my cruise more often and see what I get at different speeds.

 

[neuf]

55 due to less drag and friction, low rpms, etc, etc

 

[j_lutter31]

If you are going to go less than 55, you'll prob have roughly the same fuel consumption..(you'll just be going a shorter distance on that gallon, getting you worse mileage) This is all just theoretical talk, I dont know where that exact speed is.....its most likely your slowest speed in your top gear.........

 

[lparsons]

I was watching my tach and speedometer today on my drive in (while on the freeway). I then plotted the values against each other and found:

Which shows actually a pretty linear curve. I then corrected for speed (since I haven't changed out the speedometer gear for my 4.10 rear end gears) and did some math to arrive at revolutions per foot traveled.

Somewhat surprising is that it doesn't change much between 33 and 75 (actual speed). The absolute best was 37.5 mph (which reports as 45 on my speedometer), coming it at .717 revolutions per foot. The worst was at 75 mph (90 on my speedometer), coming it at .815 revolutions per foot. Really, only around a 13% difference in fuel economy between the two speeds.

Of course, this is all based on my eyeballing the readouts from the analog speedometer and tachometer. I suppose if we were driving OBD II cars, or had digital dashes, we could get more precise numbers.

 

[BlueOvalStangGT]

I get like 15 mpg

 

[Vipersix]

It's simply a matter of drag. Idling in your highest gear will net you the highest fuel economy. Anything above that will be using additional energy to overcome wind resistance (i.e. drag) and thus will consume more fuel.

 

[lparsons]

I think another question that should be considered in this matter is how fuel consumption relates to engine speed. I pose this question because I do not know the answer myself:

Does a single full stroke of the engine use the same volume of fuel at 4000 rpm as it does at 1000 rpm?

I guess I expected that the computer fuel injection may adjust the fuel / air mixture on the fly, but I don't know if that is true, or if so, which way that adjustment might go while running.

 

[Vipersix]

Without getting too terribly in depth with theoretical engineering, the answer to your question is yes. The Ideal Gas Law (PV=nRT) is pretty much the basis for the algorithms in the ECU. In effect, this law allows us to determine the amount of usable air in the cylinder for a given pressure and temperature (values for which our sensors measure). Since the volume is a constant, only the air density will change via differences of temperature and pressure. RPM is simply a change in time. The AFR will change on the fly many times a second for changes in any number of inputs.

One of these inputs is engine load. Engine load is directly related to your go-fast foot. Ease the aggressive nature of your go-fast foot and the engine will not experience high loads and therefore will not need high levels of fuel injection. Load is also directly related to wind resistance (though it is commonly and erroneously referenced to speed). The higher the resistance, the more power and thus fuel needed to overcome this resistance.

In sum, driving 85 will consume more fuel than driving 75 which will consume more fuel than 65 and so on down the line. However, driving 75 in fifth gear will probably get you better gas mileage than driving 55 in fourth or 35 in third. Speed is only a factor when comparing in the overdrive.

Clear as mud?

 

[94GTLaserRC]

Who cares...I dont drive a Mustang for Fuel Economy...tha't why I bought a Ford Escort.
RC

 

[fawcett]

On a 600+ mile trip I figured out that I averaged 23-25mpg. Thats with highway driving, highway traffic driving and driving through cities like NY. At 100mph in 6th gear with my 3.73s the engine spins at around 1800rpms. Getting better mpg's were not my priority. I cruised at around 75mph and that was with traffic and all of my crap from college. I was young and had more balls than brains at that time. In one section I drove at 150+mph for over 5 minutes, and when I was in Ct I got pulled over doing an honest 120mph, spoke nothing but the truth to the cop in a undercover crown vic and got off with a verbal.

Before I blew up my t5 with 2.73's, things felt really similar to my current t56 with 3.73's.
I should also note that I do not have a rear wing.
Fawcett