What's the most gas-efficient level-road cruising speed in 5th gear, please? I have a 2012 manual transmission Sport. I might guess that the hypermiling sweet spot would be at the lowest RPM (without lugging the engine), perhaps around 40 to 45 MPH. Could there be some greater fuel efficiency at a higher speed I'm not considering?

 

Watch the instant MPG meter, it will tell you. Different factors can change this such as tire construction, wind, and road surface, so it will not be a hard number and will depend on the situation.

If i'm trying to save gas I usually cruise a tad under 65 freeway and it works out for me pretty well (40mpg reliably if i'm in "eco-mode" lol best combination of not being annoyingly slow and best MPG).

 

Same here but darn near get rolled over by SUV's. People drive these things like idiots and the main reason my gas is $4 gal. Get mid 30's @ 75+mph.

 

I also have a 2012 5MT sport and I get 42-45 mpgs on flat ground going around 57 to 59mph. At that speed in 5th the engine is running just a tick below 3k rpms. That's usually the sweet spot for me but I usually can't stand going that speed on the highway for prolonged periods.

 

Dear Sy Edison,

Thanks for your answer.

By sweet spot (at about 58 MPH), do you mean peak of fuel efficiency? If so, slower (or faster) would be less fuel-efficient, right?

Are you judging by calculated miles traveled per gallon, by the instant fuel economy bar graph gage, or by the estimated average fuel economy trip gage, please?

I'd supplement my original question by assuming no rain or wind on ordinary (level and straight) interstate highway surfaces with a warmed-up engine. I realize that other factors, such as ambient air and road temperatures, tire size, composition, and inflation, oil viscosity, and fuel composition would certainly affect fuel economy. But I'm right now focused on an empirical estimate and/or theoretical guess as to the most efficient velocity, which is the factor I can most easily control from moment to moment.

Thinner oil or hyper-inflated tires may increase the fuel efficiency of my Fit, but would they change the most efficient cruising speed, please? If so, in which direction?

 

Mine seems to be 68mph but mine is a AT so RPM is quite low.

 

Dear Mini Odyssey,

At the moment of your photo, the instant fuel economy bar graph gage showed 36 MPG and the estimated average fuel economy trip gage showed 42.4 MPG since you'd last reset your trip odometer. So at that moment (going 68 MPH), you were getting fewer MPG than the average of all your speeds up to that point (since resetting), right?

 

At the time if the snap shot it was a slight hill so the bar fluctuates back n forth if you climb or decet hills. I set cruise control at 68mph on the way to work. When I leave at 5am there isn't enough traffic to make people mad driving that slow as its before rush hour. I reset my trip every full up.

 

Dear Mini Odyssey,

I'm really asking a theoretically simpler question, disregarding variables such as hills. Still, I think that your experience may be instructive. I think you're saying that if you were to set your cruise control lower (or higher) than 68 MPH, you'd get fewer MPG on your round-trip commute (hills and all), right?

If you were trying to get as far away as possible on a straight, level, smooth road with no traffic and one gallon of gas, would you accelerate gradually to 68 MPH and simply keep it there until empty?

My dad, God bless him on Fathers' Day, told me when I was a kid that the most efficient speed for gas mileage was about 35 MPH. My Fit is much lighter, is more aerodynamic, and has a much smaller and more efficient engine than the mid-1950s behemoths. But for every car, there's probably a ballpark most-efficient cruising speed.

 

Theoretically I think if the road is perfectly flat and smooth 62mph is a sweet spot but on my particular route there is a lot of mini hills and and driving any slower causes my AT to hunt up and down gears negating any potential gains so i simply drive 68 as it carries just enough momentum to go up those mini hills without hunting through gears.

 

I've been doing some research.

The sales and service departments at my local Honda dealer don't have an answer to the optimum cruising speed question. The did confirm, though, that the instant fuel economy bar graph gage and the estimated average fuel economy trip gage show results of just two bits of data they collect: from the tachometer and speedometer (plus the odometer for the average figure). They don't otherwise take into account the kind of fuel, oil, or tires, tire pressure, ambient or engine temperature, wind, rain, hills, or road surface condition. And they certainly don't measure how much fuel you're burning or have burned, which would be the real test.

I suppose that Honda engineers have built into the the gages an algorithm, such that at X RPM while traveling at Y speed yields Z gas mileage. So if you claim a certain sweet spot without actually measuring how much gas you use at that speed, you're, at best, relying on the Honda engineers' prediction. Of course, that's a practical short cut.

I've lately been watching my instant fuel economy bar graph gage, specifically with cruise control set variously at 40, 50, and 60 MPH on level roads. I find that the Honda engineers, ostensibly expressed by the gage read-outs, seem to think that there's greater efficiency at the two lower speeds than at 60. I'll have to do more experimenting to try to distinguish between 40 and 50, as they're very close.

I've found a 2009 discussion on this topic:
https://www.fitfreak.net/forums/eco-...-highways.html

There are many of online opinions on the topic, not confined, of course, to Fits. Here's one that I think is very instructive:
Does the optimal speed for fuel efficiency vary by car model? - cars fuelefficiency speedlimit | Ask MetaFilter

An important point made there is that "aerodynamic [drag] forces are proportional to the square of the speed. That means you quadruple the drag or lift when you double the speed."

And here's an erudite discussion by scientists:
Finding Optimum Cruising Speed of Car

 

I don't believe that what they're telling you is true, because with the same rpm it displays much higher mileage when going downhill.

 

Not necessarily, but in practical real world driving in my experience. You might get better efficiency going slower, but to me its not practical or safe going 40 mph on the highway when everyone else is traveling much faster.

I reset the trip meter every fill up and look at the estimated average mpg readout. I also record the number of gallons I refuel and use that sometimes.

The Fit already uses 0w oil, that's the lowest viscosity you can get. I usually have my tires pumped to 37psi.

 

This. What they said makes no sense whatsoever.

I'd trust service department guys' knowledge about as much as I would asking some random person on the street. They are salesmen, not engineers or even technicans. The last service department guy I got (not Honda) argued with me for about 30 minutes about a part replacement until I proved him wrong

It's been pretty well documented on here that the MPG meter is accurate within a few mpg and I think is accurate enough to determine most efficient cruising speed.

 

Thanks. I'm learning a lot. I now see that the instant fuel economy bar graph gauge must react to real-time data beyond inputs from the tachometer and speedometer. Could there be a fuel or air intake volume sensor involved? Does it measure the fuel injector timing, of which the gasoline used would be a function? Does it measure manifold vacuum pressure or throttle position? Maybe it measures lots of things at once and plugs them into a complicated formula.

Incidentally, why doesn't the tachometer reading dip after cresting a hill with the cruise control engaged? Of course, the RPMs plunge when I depress the clutch pedal to coast downhill. Isn't there some other trick to shut off the fuel injectors by somehow using engine breaking? If so, exactly how would I do that, please?

For the record, I don't intend to drive at slower speeds where that would be dangerous or rude. And I'm not asking the speed you normally drive. I'm now asking, "What does your instant fuel economy bar graph gauge tell you about the speed of maximum fuel efficiency on flat terrain, please?" Maybe the figures offered in MPH earlier in this thread have already directly answered that question: a) a tad under 65; b) 57 to 59; c) 62; and d) You might get better efficiency going slower [than 58]. I've reported my recent experience that 40 and 50 MPH show better efficiency than 60 MPH. I'll later try to report in further detail.

 

I spent a couple hours compiling numbers. The final answer is, SLOW. At least with a manual, there isn't a sweet spot below which you get worse mileage. (at least down to 25 mph, the lowest cruise control will engage)

With the automatic, you're limited by how slow it will go in 5th. What's that limit? It's 45 in my Odyssey. If you're dealing with hills, you're better off a little faster so it won't downshift, but on the flat, minimum speed gives you the best mileage.

I'm not saying you should drive 25 on the highway. I'm just showing the technical side. Make your own conclusions about what's practical.



Testing:
85 degrees and humid
Straight flat road, 2 miles long
Cruise control
Scangauge (calibrated) to get the numbers
Average of east and west for each data point

 

The gears are still engaged, so the rpm is fixed to vehicle speed. The fuel injectors ARE automatically shut off. The wheels and the car's momentum are driving the engine instead of the other way around. Watch the instant mileage graph. When it goes all the way across, you're in fuel cutoff mode. Nothing extra you have to do.

Also confirmed using my scangauge and watching when it goes into Open Loop mode. That means it's not using feedback from the exhaust oxygen sensors to adjust the fuel mix. There's no need to adjust the mix when it's at zero fuel.

 

Yes, engine breaking will shut off the injectors, so when you're coming downhill go ahead and leave the clutch out. When you put the clutch it the car will restart the injectors to maintain idle.

There are some good threads on here about how the car calculates MPG, it is a formula, but i'm not sure what the variables are other than it calculates engine load and speed (not sure what sensors it uses).

I say for speed "a little under 65" because that's about the slowest I can go here on the freeways in the right lane. On the street if i'm cruising for MPG i'll stick around 40 almost coasting in 5th only occasionally pressing the gas a tiny bit to keep speed, pulse and glide.

 

Dear PaleMelanesian,

Science! Data! You're the best! That's a lot of work. Thank you.

I'm fascinated and surprised that your data shows efficiency increases as low as 25 MPH (and maybe beyond). But if the major factor of speed is aerodynamic drag, I suppose that makes some sense.

I'm also intrigued at how each gear's profile is so linear and that those lines are almost exactly parallel to that of the other gears. All the marching band members are perfectly lined up on the football field.

The one data point for 5th gear with A/C seems to reflect about a 10% (4.5 MPH) decrease in fuel efficiency. Maybe additional data points would describe a similarly-parallel line.

I think I see roughly a 7.5 MPH increase in 5th gear efficiency for every 10 MPH below 70 MPH. It may even make sense to extrapolate in the opposite direction along the same line up to 80 or 90 MPH to 29 and 21 MPG, respectively.

Thanks for explaining why RPMs don't dip when going downhill with gears engaged.

The next logical question is about hill climbing. Is there a reason to think that your level-road data wouldn't translate to a greater efficiency in driving slowly--within the useful range of the highest gear that will make it--when going uphill?

 

Thanks! It cost me a gallon and a half of gas to get this. I'm glad it's useful to you. I was planning to do this already, but your question made me do it sooner.

I expect the lines would continue to be linear, so your extrapolations make sense.

AC - maybe parallel, or maybe not. AC is pretty much a fixed load at all speeds, added on top of a varying driving load. I'm not sure how that would translate.

Climbing - you're right. As long as it can maintain speed, lower is better uphill. (from experience of several years of hypermiling with a scangauge)